Endometriosis can occur in men, though it is extraordinarily rare. The condition, defined by the presence of endometrial-like tissue outside the uterus, has been documented in the male lower genitourinary tract and abdominal cavity in a small number of case reports spanning several decades.1PubMed Central. Endometriosis in a Man as a Rare Source of Abdominal Pain: A Case Report and Review of the Literature The fact that it exists at all in people without a uterus challenges long-held assumptions about the disease and raises questions about what actually drives it.
What Male Endometriosis Looks Like
The handful of documented cases tend to follow a similar pattern. A man shows up with unexplained abdominal or pelvic pain, imaging reveals a mass nobody expected, and the tissue is surgically removed. Only after the pathologist examines it under a microscope does the diagnosis land. In one well-documented case, a 40-year-old man visited his doctor for abdominal pain, imaging found a midline mass, and surgical removal followed by histological analysis confirmed endometriosis.1PubMed Central. Endometriosis in a Man as a Rare Source of Abdominal Pain: A Case Report and Review of the Literature In another case, a male patient presented with an unusual abdominal mass that, on microscopic examination, showed thick smooth muscle fibers surrounding a central lumen bordered by endometrial glands and stroma.2PubMed Central. An unusual cause of abdominal pain in a male patient: Endometriosis
Unlike endometriosis in women, where cyclical pelvic pain, painful periods, and infertility are the usual flags, symptoms in men tend to be vague. Abdominal pain, urinary problems, or a palpable mass are the most commonly reported complaints. There is no cyclical pattern to the pain, which makes clinical suspicion even lower. The bladder, the lower abdomen along the midline, and the area around the prostate are the sites where male endometriosis has most often turned up. A doctor evaluating these symptoms in a male patient would almost never suspect endometriosis initially, which is part of why every confirmed case has been a surprise diagnosis made after surgery.
Why Men Have the Tissue in the First Place
The existence of endometrial tissue in a male body sounds like it should be impossible, but embryology offers a plausible explanation. During early fetal development, all embryos have both Müllerian ducts (which typically develop into the uterus, fallopian tubes, and upper vagina) and Wolffian ducts (which typically develop into male reproductive structures). In male embryos, the Müllerian ducts usually regress almost completely. “Almost” is the key word. Tiny remnants of Müllerian tissue can persist even in genetically male individuals. These remnants are microscopic and usually harmless, but under certain conditions they can become the seed from which endometrial-like tissue grows.
Two cases of endometriosis with cystic changes in males were found along the anatomical route where the Müllerian duct would have existed during development, supporting the theory that these lesions arise from embryonic remnants.3PubMed. Endometriosis With Cystic Degeneration: A Rare Disease of Males The location of the tissue essentially traces the path where those ducts ran before they were supposed to disappear. This embryonic-remnant theory is the most widely accepted explanation for male endometriosis, though there is a second possibility worth mentioning: metaplasia, where one type of adult tissue transforms into another under the right stimulus. Some researchers think that peritoneal cells lining the abdominal cavity could, when exposed to certain hormones, transform into endometrial-like tissue regardless of the person’s sex. Both mechanisms could potentially be at work in different cases.
The Role of Estrogen
If dormant embryonic remnants are the fuel, estrogen appears to be the spark. The most consistent risk factor across reported cases of male endometriosis is prolonged exposure to estrogen or estrogen-like compounds.1PubMed Central. Endometriosis in a Man as a Rare Source of Abdominal Pain: A Case Report and Review of the Literature This makes biological sense. In women, endometriosis is an estrogen-dependent disease; lesions grow and become symptomatic in the presence of estrogen and tend to quiet down after menopause when estrogen levels drop. The same hormonal dependency appears to hold in men.
The clearest illustration comes from prostate cancer treatment. One documented case involved a man who developed bladder endometriosis after receiving long-term estrogen therapy for prostate cancer.4PubMed. Bladder endometriosis developed after long-term estrogen therapy for prostate cancer Estrogen was historically used to suppress testosterone in prostate cancer patients (a practice largely replaced by other drugs today but still occasionally encountered). The years-long estrogen exposure in this patient created the hormonal environment in which latent endometrial tissue could activate and grow. Another case involved a man with liver cirrhosis who was taking spironolactone, a medication that can have anti-androgenic and estrogenic effects. That patient developed an “endomyometriosis” or uterus-like mass, and researchers suggested the combination of cirrhosis (which impairs the liver’s ability to metabolize estrogen) and spironolactone created the altered hormonal state that triggered the condition.5PubMed. Endomyometriosis (“Uterus-like mass”) in an XY Male: Case Report With Molecular Confirmation and Literature Review
The pattern across cases is consistent: something shifts the hormonal balance toward estrogen dominance, and tissue that would otherwise sit quietly for a lifetime begins to behave like functional endometrium. The man doesn’t need to have high absolute estrogen levels by female standards. What seems to matter is a relative shift, enough estrogen activity to wake up tissue that was never supposed to be active.
How the Diagnosis Gets Confirmed
No blood test or imaging scan can definitively diagnose endometriosis in men. The diagnosis is made under the microscope after tissue is removed. In the case reported in the Avicenna Journal of Medicine, the pathologist found endometrial glands and stroma surrounded by smooth muscle, and immunostaining showed the tissue was positive for estrogen receptors and progesterone receptors in the glandular component, and for CD10 in the stroma.2PubMed Central. An unusual cause of abdominal pain in a male patient: Endometriosis Those markers are the same ones pathologists use to confirm endometriosis in women. Finding estrogen and progesterone receptors in the tissue reinforces the hormonal connection and also has treatment implications, because it means the tissue will respond to hormonal manipulation.
The diagnostic challenge is not really about what to look for under the microscope. It is about getting there in the first place. Before surgery, these cases are typically misdiagnosed or categorized as “abdominal mass of unknown origin.” Imaging may show a cystic or solid mass, but nothing about the radiological appearance screams endometriosis in a male patient. Clinicians working up an abdominal mass in a man are thinking about tumors, abscesses, hernias, or developmental anomalies. Endometriosis does not usually make the list of possibilities, so it is only discovered incidentally when pathology results come back.
Treatment When It Does Occur
Treatment for male endometriosis generally involves two approaches: surgery and hormonal management. In documented cases, surgical removal of the endometriotic mass has been the primary intervention, and stopping estrogen exposure (when it was the trigger) has been the other critical step. If symptoms develop, cessation of estrogen therapy combined with careful surgical intervention can successfully relieve them.1PubMed Central. Endometriosis in a Man as a Rare Source of Abdominal Pain: A Case Report and Review of the Literature
The logic is straightforward. If the lesions are hormone-responsive (and the immunostaining suggests they are), removing the hormonal fuel should stop progression or cause regression. For a man on estrogen therapy for prostate cancer, this could mean switching to a different class of anti-androgen. For someone with liver disease affecting hormone metabolism, managing the underlying liver condition becomes part of the endometriosis strategy. In women, aromatase inhibitors have been investigated as a way to cut off local estrogen production in endometriotic lesions, but the evidence for their efficacy remains weak and they have not been studied specifically in male cases.6PubMed. Are aromatase inhibitors effective in endometriosis treatment?
Given how few male cases exist, there are no clinical guidelines for treating this population. Each case is essentially managed individually based on general principles borrowed from female endometriosis treatment: remove the lesion if it’s causing symptoms, and reduce estrogen exposure where possible. Long-term follow-up data is nonexistent, so whether these lesions tend to recur in men after treatment is something nobody can answer with confidence.
Endometriosis in Transgender Men
A related but distinct conversation has emerged around endometriosis in transgender men. This is a different situation from endometriosis in cisgender males. Transgender men who were assigned female at birth have a uterus and endometrial tissue, so they can develop endometriosis through the same pathways as cisgender women. The question is whether testosterone therapy, which most transgender men take as part of gender-affirming care, protects against or complicates the condition.
Endometriosis has been recognized as a significant issue for transgender men, with diagnostic difficulties arising because hormone therapy and surgical interventions can alter the typical symptom picture.7PubMed Central. Endometriosis in transgender men: recognizing the missing pieces A systematic review pooling data from over 1,500 transgender men found a prevalence of endometriosis around 9%, with roughly 40% of those diagnosed having early-stage disease.8Journal of Minimally Invasive Gynecology. Endometriosis in Transgender Men: A Systematic Review That prevalence is lower than estimates for the general population of people with uteruses, but the comparison is complicated by the fact that many of these individuals were diagnosed incidentally during hysterectomy rather than from symptomatic investigation.
Despite testosterone’s suppressive effects on the endometrium, it does not eliminate endometriosis risk. Over half of transgender men in the review reported non-menstrual pelvic pain, and about a quarter reported painful periods even while on testosterone.8Journal of Minimally Invasive Gynecology. Endometriosis in Transgender Men: A Systematic Review The disease can persist or even progress during testosterone therapy, possibly because the body converts some testosterone into estrogen locally through aromatase activity within endometriotic tissue itself. This is an area where the limited research is a real problem. Transgender men who report pelvic pain may face barriers to diagnosis, including reluctance to seek gynecological care, clinician unfamiliarity with the presentation, and the assumption that testosterone should have resolved any endometrial issues.
What These Cases Reveal About Endometriosis as a Disease
Male endometriosis, rare as it is, has outsized importance for understanding the disease in general. For decades, endometriosis was defined almost entirely in the context of menstruation and the female reproductive system. The prevailing early theory, retrograde menstruation (where menstrual blood flows backward through the fallopian tubes and deposits endometrial cells in the pelvis), was treated as the primary explanation for how endometriosis starts. Male cases quietly dismantle that framework. Men do not menstruate, do not have fallopian tubes, and do not have a uterine cavity from which cells could reflux. The fact that they can still develop histologically confirmed endometriosis means the disease has mechanisms that extend beyond menstrual backflow.
The embryonic-remnant and metaplasia theories that explain male cases have gained broader traction in the field as a result. Researchers now recognize that endometriosis is likely driven by multiple pathways, and that menstrual reflux, while it may contribute in many female cases, is probably not the whole story even in women. Conditions where endometrial tissue appears in locations far from the pelvis (the lungs, the diaphragm, and now the male urogenital tract) all point toward mechanisms like lymphatic or vascular spread, or transformation of local tissue under hormonal influence.
Why Awareness Matters Even for Something This Rare
The natural reaction to something this uncommon is to file it under medical curiosities and move on. But for the men who actually develop these lesions, awareness can mean the difference between years of unexplained symptoms and a correct diagnosis. Abdominal pain in men has a long differential diagnosis, and endometriosis is virtually never on it. If a man has been on long-term estrogen therapy, has significant liver disease affecting hormone metabolism, or is taking medications with estrogenic effects, and he develops unexplained pelvic or lower abdominal symptoms, the possibility of endometriosis deserves at least a moment of consideration from his medical team.
There is also a broader point about how gendered assumptions shape medical thinking. Endometriosis already takes an average of several years to diagnose in women, partly because pain symptoms are normalized or dismissed. In men, the condition is not even in the diagnostic vocabulary. Every confirmed male case was an accidental finding after surgery performed for a different suspected diagnosis. Building the condition into clinical awareness, even as a remote possibility in the right clinical context, is the only way to move from surprise diagnoses to intentional ones.
Conditions That Mimic Endometriosis in Men
Before a pathologist confirms endometrial glands and stroma under the microscope, several other conditions look nearly identical on imaging or during initial evaluation. An abdominal cystic mass in a man could be a mesenteric cyst, a remnant of other embryonic structures like the urachus, a dermoid cyst, or a soft-tissue tumor. Bladder lesions that turn out to be endometriosis are often initially suspected to be bladder cancer, which is far more common. A mass along the midline of the lower abdomen might be investigated as a possible sarcoma or lymphoma before anyone thinks about benign endometrial tissue.
This overlapping differential diagnosis is another reason these cases are always diagnosed after surgery. Pre-operative biopsy of deep abdominal or pelvic masses is not always practical, and even when it is, the pathologist needs a high-quality tissue sample and the clinical suspicion to run the right immunostains. Without specifically looking for endometrial markers like estrogen receptors, progesterone receptors, and CD10 in the stroma, a small tissue sample could be misclassified. The message for clinicians is less “suspect endometriosis” and more “when a mass in a male patient doesn’t match any expected diagnosis, consider sending the tissue for a broader panel of stains.” The pathology almost always provides the answer if the right questions are asked.
The Numbers in Perspective
It is worth being clear about how rare this truly is. There is no population-based prevalence figure for male endometriosis because the number of confirmed cases across the entire medical literature can essentially be counted by hand. We are talking about individual case reports and small case series, not epidemiological data. The literature reviews that exist tend to compile fewer than two dozen reported cases spanning multiple decades, many of them occurring in the setting of exogenous estrogen therapy or liver disease. A man without those specific risk factors developing endometriosis spontaneously appears to be vanishingly rare, though it is impossible to know whether some cases go undiagnosed because nobody suspects the condition.
Contrast this with endometriosis in cisgender women, which affects roughly one in ten women of reproductive age. The two populations are separated by orders of magnitude in frequency. Male endometriosis is not a hidden epidemic waiting to be uncovered. It is a biological curiosity with genuine clinical relevance for the tiny number of individuals it affects, and genuine scientific relevance for what it teaches about disease mechanisms. The condition reminds us that biological sex is a spectrum at the cellular level even when it appears binary at the anatomical level. Remnant tissues, hormone fluctuations, and cellular plasticity create a landscape where “impossible” diagnoses occasionally turn out to be real.