Your flank is the area on each side of your torso between the lower edge of your ribcage and the top of your hip bone, wrapping from the side of your abdomen around toward your spine. Most people instinctively touch the right spot when they have a dull ache “in their side,” but the flank is more precisely defined than casual use suggests, and its anatomy matters because several organs sit just beneath it. Understanding where the flank actually starts and stops helps make sense of why doctors ask pointed questions when you report pain there.
The Boundaries of the Flank
The flank occupies a band of your torso that is bordered above by the margin of your lowest ribs (roughly the 11th and 12th), below by the iliac crest (the curved rim of your pelvis you can feel at your hip), in front by the lateral edge of your abdominal muscles, and in back by the erector spinae muscles that run alongside your spine. Surgeons sometimes describe the region as confined by the costal margin, spine, and pelvic brim, a triangular zone that presents unique challenges during hernia repair because of its proximity to bony landmarks and the nerves running through the lateral abdominal wall.1PubMed Central. Flank and Lumbar Hernia Repair If you stand up and place your hands on your waist with fingers pointing backward, your palms are roughly over the flank.
The region sits over three layers of lateral abdominal muscle: the external oblique, the internal oblique, and the transversus abdominis. Those muscle layers are thinner here than in the front of your abdomen, and the connective tissue between them is less reinforced, which is one reason hernias and post-surgical bulging can develop in the flank more easily than in the midline of your belly.
What Sits Beneath the Flank
The kidneys are the headliner organs of the flank. Each kidney is a fist-sized organ tucked against the posterior abdominal wall at about the level of the 12th rib, partially shielded by the lower ribs and cushioned by a layer of fat called the perirenal fat. Because the kidneys are retroperitoneal, meaning they sit behind the membrane lining the abdominal cavity rather than inside it, they are closer to your back than to your belly button. That is why kidney-related pain is usually felt in the flank and back rather than up front.
On the right side, the lower pole of the kidney neighbors the ascending colon and sits just below the liver. On the left, the kidney is near the descending colon and the tail of the pancreas. The spleen tucks under the left ribcage just above the kidney. The adrenal glands perch on top of each kidney like small caps. All of these structures can, when diseased or injured, generate pain that a patient describes as flank pain, which is part of what makes the symptom tricky to interpret.
Why Doctors Pay Attention to Flank Pain
Flank pain is one of the most common complaints that brings people to emergency departments, and the reflex assumption is usually that it involves the kidneys or urinary tract. That instinct is often correct. Kidney stones lodged in the ureter cause a sharp, cramping pain that radiates from the flank toward the groin. Acute pyelonephritis, a bacterial kidney infection, produces a deep flank ache accompanied by fever and sometimes nausea. Ultrasound is a first-line tool for sorting out these causes: in pyelonephritis the affected kidney may appear enlarged with changes in its internal echo pattern, though clinical and laboratory findings remain essential for a confident diagnosis.2Ultrasonography. Ultrasonography of acute flank pain: a focus on renal stones and acute pyelonephritis
One classic bedside test is checking for costovertebral angle (CVA) tenderness. The costovertebral angle is the point where your lowest rib meets your spine on each side, right over the kidney. A doctor taps firmly with the heel of their hand over that angle; if the tap reproduces a sharp pain, it suggests the kidney or its surrounding tissues are inflamed. Research has found that the distance from skin to the renal pelvis matters for this test: in patients with thicker tissue overlying the kidney, the tap may not provoke tenderness even when an infection is present, making CVA tenderness a less reliable indicator in those individuals.3PubMed. Skin-to-Renal Pelvis Distance Predicts Costovertebral Angle Tenderness in Adult Patients with Acute Focal Bacterial Nephritis
Musculoskeletal Sources of Flank Pain
Not every ache between your ribs and hip involves an organ. The lateral abdominal muscles themselves can be strained, though it happens less commonly than strains of the front-facing muscles. A case involving a professional soccer player illustrated this well: he felt a popping sensation with sharp pain localized between the 11th and 12th ribs, and ultrasound confirmed an injury to the external oblique muscle with fluid collecting beneath the rib.4PubMed Central. Uncommon external abdominal oblique muscle strain in a professional soccer player: a case report Sports that involve sudden trunk rotation, like tennis, golf, and soccer, put the oblique muscles at risk for this kind of strain.
Rib fractures at the 11th or 12th rib can mimic kidney pain convincingly, particularly when there is no obvious trauma history. Lower rib fractures sometimes occur from chronic coughing, osteoporosis, or repetitive motion, and because the pain sits right in the flank zone, both the patient and the initial examiner may first suspect a renal cause. Costochondritis or intercostal muscle irritation can also produce a nagging flank ache that worsens with twisting movements.
Nerve-Related and Skin-Level Causes
Herpes zoster, better known as shingles, frequently targets the thoracic dermatomes that wrap around the trunk at flank level. The hallmark is a band of painful, blistering rash that follows the path of a single nerve from the spine around toward the front of the body. Early on, before the rash appears, the burning or stabbing pain can be mistaken for kidney or muscle trouble. The diagnosis becomes clearer once the characteristic skin lesions and dermatomal pain pattern emerge.5Neuropathic Pain. Herpes Zoster and Postherpetic Neuralgia After the rash heals, some people develop postherpetic neuralgia, a persistent pain in the same distribution that can last months or years.
Diabetic thoracic radiculopathy is another nerve-related cause. Damage to thoracic nerve roots from long-standing diabetes can produce a burning sensation across the flank and abdomen on one side. It is uncommon enough that it often goes undiagnosed for some time, with patients undergoing abdominal workups before the neurological source is identified.
Dangerous Causes That Mimic Routine Flank Pain
The biggest reason clinicians take flank pain seriously even when the most likely cause is a kidney stone is that a small percentage of cases turn out to involve life-threatening conditions. A ruptured abdominal aortic aneurysm can present with flank pain that looks, at first glance, like a renal or musculoskeletal problem. When blood leaks from a ruptured aneurysm into the retroperitoneal space, it irritates the same tissues that kidney problems irritate, and the pain may radiate into the groin or hip. Symptomatic aortic aneurysms typically present with pain localized to the abdomen, back, or flank, and a missed diagnosis carries a mortality rate as high as 78 to 94 percent in ruptured cases.6PubMed Central. An unusual presentation of ruptured abdominal aorta aneurysm7International Journal of Surgery Case Reports. Symptomatic abdominal aortic aneurysm (AAA) presenting as unilateral lower limb swelling: A case report
Retroperitoneal hemorrhage from other causes, such as a bleeding adrenal gland, spontaneous kidney bleeding, or complications of blood-thinning medication, can also produce acute flank pain. One visible clue that internal bleeding has tracked into the flank tissues is the Grey Turner sign: bruising or discoloration along the flanks. Grey Turner sign, sometimes seen alongside Cullen sign (bruising around the belly button), indicates that blood has dissected through tissue planes and warrants urgent investigation with imaging and blood work, including coagulation studies and pancreatic enzyme levels.8CMAJ. Cullen and Grey Turner signs in idiopathic perirenal hemorrhage The sign has been described in acute pancreatitis, splenic rupture, ruptured ectopic pregnancy, and as a complication of anticoagulation therapy.
Flank Pain During Pregnancy
Pregnant women commonly report flank discomfort, and the assumption is often that the growing uterus is compressing a ureter and causing hydronephrosis, the backup of urine into the kidney. Hydronephrosis does develop in roughly a fifth of pregnancies, predominantly on the right side and mostly during the third trimester. But the connection between flank pain and hydronephrosis during pregnancy turns out to be surprisingly weak. In one prospective study, 17 of 81 pregnant women had some degree of hydronephrosis, yet there was no meaningful correlation between reporting significant flank pain and actually having hydronephrosis. Of the women with hydronephrosis, most had no flank pain at all, and several women with the worst flank pain had perfectly normal kidneys on ultrasound.9American Journal of Obstetrics & Gynecology. Maternal hydronephrosis in pregnancy: Poor correlation with clinical symptoms10PubMed. Maternal hydronephrosis in pregnancy: poor association with symptoms of flank pain
The practical takeaway for pregnant patients and their providers is that flank pain during pregnancy should not automatically be chalked up to hydronephrosis. Other causes, including kidney stones (which do occur in pregnancy), musculoskeletal strain from postural changes, and urinary tract infections, need to be evaluated. Reassuringly, pregnancy-related hydronephrosis resolves on its own, typically within about six weeks after delivery.
Surgical Approaches to the Flank
Surgeons access the flank for procedures on the kidneys, adrenal glands, and retroperitoneal structures. The standard retroperitoneal flank incision gives a direct route to the kidney without entering the peritoneal cavity, which reduces the risk of bowel-related complications. However, this incision carries its own trade-off: injury to the intercostal nerves that run along the undersides of the lower ribs. When those nerves are damaged, the abdominal wall muscles they supply can weaken, producing a visible bulge in the flank that patients sometimes mistake for a hernia. Research has established that this postoperative bulge is related to nerve injury causing paralysis of the local musculature, and that avoiding extension of the incision into the 11th intercostal space significantly reduces the risk.11PubMed. The retroperitoneal incision. An evaluation of postoperative flank ‘bulge’
Flank and lumbar hernias, whether they occur spontaneously, after surgery, or following trauma, are relatively uncommon compared to groin or midline abdominal hernias. Repairing them is considered more difficult because the flank lacks a flat, muscular surface to anchor a repair. The bony landmarks on all sides, the rib margin above and the pelvic rim below, limit how a surgical mesh can be laid and fixed. These factors explain why flank hernia repairs have historically had higher recurrence rates than inguinal hernia repairs, and why they often require specialized surgical planning.1PubMed Central. Flank and Lumbar Hernia Repair
The Flank in Cosmetic Surgery
Outside of medical contexts, many people encounter the word “flank” in cosmetic surgery discussions. The flanks are one of the most commonly targeted areas for liposuction, often referred to colloquially as “love handles.” But the fat in the flank behaves differently from fat elsewhere on the trunk. It is denser, more adherent to the underlying tissue, and located deeper than the comparatively softer, more fluid fat on the front of the abdomen. These characteristics make liposuction of the flank technically more demanding. The proximity of the kidneys to the posterior flank also introduces a safety concern: aggressive deep liposuction in this region carries a higher risk of retroperitoneal perforation compared to anterior abdominal liposuction.12Revista Brasileira de Cirurgia Plástica. Flankplasty systematization based on surgical anatomy
Flankplasty, a procedure that excises skin and fat from the flank, is sometimes used as an alternative or complement to liposuction, particularly in patients who have lost significant weight and have excess, hanging skin. Because the flank’s fat is more fibrous, surgeons sometimes find that direct excision achieves a more predictable contour than suction alone. The anatomical quirks of the region, thin muscle, dense fat, nearby organs, and limited bony support, all factor into surgical planning whether the goal is medical or cosmetic.
How to Describe Flank Pain to Your Doctor
If you are trying to communicate flank pain accurately, a few details help your doctor narrow down the cause faster. Location matters more than you might think: pain closer to the spine suggests the kidney or paraspinal muscles, while pain more toward the front of the flank may involve the colon, abdominal wall muscles, or a hernia. Whether the pain is constant or comes in waves is another useful detail. Colicky pain, the kind that builds to a peak and then fades before returning, strongly suggests a kidney stone or ureteral obstruction. A steady, deep ache with fever points more toward infection.
Radiation of the pain helps too. Kidney stone pain classically radiates from the flank downward toward the groin and inner thigh on the same side. Musculoskeletal pain tends to worsen with specific movements and may be reproducible by pressing on a particular spot. Pain that is positional, worse when lying on one side or better when curling up, provides a different set of clues. Noting whether you have any associated symptoms such as blood in the urine, nausea, fever, or changes in urination gives your provider context that imaging alone cannot always supply.
One underappreciated point: flank pain on the right side and flank pain on the left side do not carry identical lists of possible causes, because the organs underneath differ. Right flank pain brings the liver, gallbladder, and ascending colon into the picture. Left flank pain involves the spleen, descending colon, and tail of the pancreas. Both sides share the kidneys, adrenal glands, and musculoskeletal structures, but the asymmetry of your internal anatomy means that the side of your pain is a genuine diagnostic clue, not just a detail to mention in passing.