A black line running lengthwise down your toenail is usually a streak of melanin pigment trapped in the nail plate as it grows, a condition doctors call longitudinal melanonychia. In most people, the cause is benign: normal melanocyte activity, minor trauma, a fungal infection, or simply having darker skin. But the same appearance can, rarely, signal subungual melanoma, a form of skin cancer that grows beneath the nail and is frequently misdiagnosed as something harmless. Understanding what to look for and when to seek evaluation matters because early detection dramatically changes outcomes.
What Melanonychia Actually Is
Melanonychia describes a black, brown, or gray pigmentation of the nail plate that usually shows up as a vertical band stretching from the base of the nail toward the tip. It can appear on any finger or toe, on a single nail or several at once, and at any age.1PubMed Central. Use of Nail Dermoscopy in the Management of Melanonychia: Review The pigment itself can come from outside the body (bacteria, fungi, dried blood, or even tar) or from within, when melanocytes in the nail matrix ramp up melanin production and deposit it into the growing nail plate.2PubMed. Longitudinal melanonychias Because so many different conditions share this single visible feature, working out the actual cause depends on the band’s color, width, edges, and whether it is changing over time.
The Most Common Benign Causes
By far the most frequent reason for a dark longitudinal band is what dermatologists call melanocytic activation: the melanocytes already sitting in your nail matrix simply start producing more pigment. In people with darker skin tones, this is so common that it has its own name, ethnic melanonychia, and is considered a normal variant rather than a medical problem.3PubMed. Melanonychia striata: clarifying behind the Black Curtain. A review on clinical evaluation and management of the 21st century Longitudinal melanonychia is seen far more often in non-Caucasian populations than in Caucasian ones.4PubMed. Key point in dermoscopic differentiation between early nail apparatus melanoma and benign longitudinal melanonychia
Beyond ethnicity, a long list of triggers can switch on melanocyte activity. Pregnancy, chronic friction or trauma to the toe (think tight shoes or repetitive running impact), certain medications, and systemic conditions like endocrine disorders or hemochromatosis can all produce dark streaks.3PubMed. Melanonychia striata: clarifying behind the Black Curtain. A review on clinical evaluation and management of the 21st century Benign growths within the nail matrix, like melanocytic nevi (essentially moles under the nail) and lentigines (small pigmented spots), account for another set of cases where the melanocytes themselves have multiplied but remain entirely harmless.
Subungual Hematoma and Other Trauma
A sudden dark discoloration under the toenail is often just trapped blood from an injury you may not even remember. Stubbing your toe, dropping something on it, or wearing shoes that repeatedly jam your toenail can all cause a subungual hematoma. These tend to look more purple, red-black, or splotchy rather than forming a neat longitudinal band, and they shift position as the nail grows out.5Dermatology. Examination of Subungual Hematomas and Subungual Melanocytic Lesions by Using Optical Coherence Tomography and Dermoscopy Under dermoscopy, hematomas show characteristic homogeneous patterns, globular shapes, and peripheral fading that help clinicians tell them apart from pigmented growths.6British Journal of Dermatology. Dermoscopy of subungual haemorrhage: its usefulness in differential diagnosis from nail‐unit melanoma
The practical clue for you is growth: a hematoma moves distally (toward the tip of the toe) over weeks to months as the nail grows. A pigmented band caused by active melanocytes stays anchored at the base. If a dark mark under your nail is slowly migrating outward and fading, it is almost certainly old blood working its way out.
Infections That Darken the Nail
Fungal infections (onychomycosis) are a well-known source of nail discoloration, typically yellowing or whitening, but certain fungi and bacteria can produce dark pigments that mimic melanonychia. One of the more distinctive culprits is Pseudomonas, a bacterium that thrives in moist environments and colonizes damaged or lifted nail plates. The resulting “green nail” can range from pale green to dark green verging on black.7PubMed. Green nail: Etiology and treatment of chloronychia When the color skews very dark, it is easy to confuse a bacterial infection with something more worrying.
This overlap matters because subungual melanoma itself has been misdiagnosed as a fungal infection. One published case involved a 48-year-old man whose melanoma of the third toenail was initially treated as onychomycosis, delaying the correct diagnosis.8Medical Research Archives. Subungual Melanoma of the Toenail Presenting as Dystrophic Discoloration: A Case Report A general rule: if an apparent infection is not responding to treatment, or if the discoloration is specifically a longitudinal band rather than diffuse nail damage, a closer look is warranted.
Medications and Chemotherapy
Certain drugs can trigger melanonychia as a side effect by stimulating melanocytes in the nail matrix. Chemotherapy agents are particularly well known for causing nail changes, which can involve all nails or just a few.9PubMed. Nail toxicity induced by cancer chemotherapy Combination chemotherapy regimens are especially associated with simultaneous lightening and darkening of nails, where melanonychia may coexist with white nail changes.10PubMed Central. Review Narrative Review of Drug-Associated Nail Toxicities in Oncologic Patients Antimalarials, certain antibiotics, and antiretroviral medications can also do this. If your dark nail streak appeared around the same time you started a new medication, that connection is worth mentioning to your doctor. Drug-induced melanonychia usually affects multiple nails and fades after the medication is stopped.
When to Worry About Melanoma
Subungual melanoma is rare, but it is the reason this topic gets serious attention. It accounts for a small fraction of all melanomas, yet it carries a disproportionately poor prognosis because it is so often caught late. An estimated 85% of cases are misdiagnosed at the time of initial presentation, and the reported five-year survival rate is only about 27%.11PubMed Central. A Misdiagnosed Case of Malignant Melanoma in an Infected Nail: A Case Report The conditions most commonly mistaken for it include fungal infections, subungual hematomas, bacterial infections, and benign nevi.
Several features should raise your suspicion. A dark band that is new, that appears on only one nail, that is widening over time, or that has blurry or irregular edges deserves professional evaluation. Pigmentation that extends beyond the nail itself onto the surrounding skin of the cuticle or nail fold is called Hutchinson’s sign and has long been considered an important red flag for subungual melanoma.12PubMed. Hutchinson’s sign: a reappraisal In one study of over 400 patients, Hutchinson’s sign was present in roughly 83% of subungual melanoma cases, and was associated with pigmentation extending beyond half the nail width, pigmentation wider than the longitudinal band itself, and a discontinuous pattern.13PubMed. Clinical differences between Hutchinson’s sign in subungual melanoma and pseudo-Hutchinson’s sign in benign longitudinal melanonychia
That said, Hutchinson’s sign is not a perfect alarm. A scoping review found the sign present in fewer than half of nail melanoma cases examined dermoscopically, underscoring that its absence does not rule out cancer.14Clinical and Experimental Dermatology. Dermoscopic features of nail unit melanoma: a scoping review And a lookalike phenomenon called pseudo-Hutchinson’s sign can appear in benign melanonychia: in the same study mentioned above, about 45% of benign cases showed periungual pigmentation that mimicked the real thing.13PubMed. Clinical differences between Hutchinson’s sign in subungual melanoma and pseudo-Hutchinson’s sign in benign longitudinal melanonychia This is exactly why clinical judgment and additional tools matter more than any single visual clue.
How Doctors Tell Benign From Dangerous
The first tool beyond a visual exam is dermoscopy, a technique where a dermatologist uses a magnifying device with polarized light to examine the nail plate and the pigmented band in detail. In subungual hematomas, dermoscopy typically reveals purple and red-black hues with homogeneous or globular patterns and fading at the edges. Melanomas, by contrast, tend to show irregular longitudinal lines or bands, a triangular shape widening toward the cuticle, and sometimes ulceration.6British Journal of Dermatology. Dermoscopy of subungual haemorrhage: its usefulness in differential diagnosis from nail‐unit melanoma None of the classic melanoma features appeared in subungual hematoma cases in that study, which gives dermoscopy strong discriminating power between the two.
When dermoscopy leaves uncertainty, or when the clinical picture is worrying enough, the next step is a nail matrix biopsy. This involves removing a small sample of tissue from the nail matrix, which is the root area where the nail grows and where melanocytes live. Several techniques exist, including punch biopsy, elliptical excision, and tangential (shave) excision.15PubMed Central. Nail Biopsy: A User’s Manual The procedure is done under local anesthesia. A biopsy is the only way to definitively confirm or rule out melanoma.
The Problem of Darker Skin and Overdiagnosis
People with darker skin tones face a frustrating double bind. Benign melanonychia is extremely common in this group, yet the clinical features of benign bands in darker-skinned patients tend to look more alarming by conventional assessment criteria. Research has shown that darker-skinned patients have wider bands with lower brightness, more band changes over time, and are more likely to undergo biopsy than lighter-skinned patients with benign melanonychia.16PubMed. Clinical and dermoscopic findings of benign longitudinal melanonychia due to melanocytic activation differ by skin type and predict likelihood of nail matrix biopsy This means clinicians trained primarily on lighter-skinned populations can mistake normal ethnic melanonychia for something suspicious, leading to unnecessary biopsies and patient anxiety.
On the flip side, dismissing every dark band in a dark-skinned patient as “just ethnic melanonychia” carries its own danger, because subungual melanoma does occur in people of all skin tones and is actually a larger proportion of all melanoma diagnoses in Black and Asian populations. The key difference is context: ethnic melanonychia tends to appear on multiple nails, remains stable in width and color, and often has a family pattern. A new, solitary, widening, or irregularly colored band in anyone, regardless of skin tone, warrants closer attention.
Black Lines on Children’s Toenails
Longitudinal melanonychia in children is uncommon, and most cases turn out to be benign melanocytic nevi or lentigines. Subungual melanoma is exceedingly rare in the pediatric population, but it cannot be entirely dismissed.17PubMed Central. Pediatric longitudinal melanonychia What makes evaluation tricky in children is that benign nevi under the nail can change more dynamically during growth spurts, widening or darkening in ways that would look suspicious in an adult. Pediatric dermatologists generally recommend careful clinical monitoring, sometimes aided by dermoscopy, before jumping to biopsy. The threshold for biopsy is usually higher in children unless the band is rapidly changing, very wide, or accompanied by pigment spilling onto the surrounding skin.
Treatment When Melanoma Is Confirmed
For decades, the standard treatment for subungual melanoma was amputation of the affected digit. That approach guaranteed clear margins but came at an obvious functional and psychological cost, especially for finger melanomas. More recent evidence has shifted the thinking. A review of the literature found that amputation did not prove significantly better than more conservative wide local excision in terms of survival or recurrence for thinner tumors.18PubMed. Subungual melanoma: a review of current treatment For melanoma in situ (confined to the top layer, with no invasion into deeper tissue), wide excision of the nail unit is now considered appropriate treatment.
Functional, digit-preserving surgery has gained ground for thin invasive melanomas as well. One case series found that wide local excision was associated with low recurrence and better functional outcomes than amputation, and could be considered for tumors up to about 4 mm thick, provided the surgical margins were carefully examined.19Actas Dermo-Sifiliográficas (English Edition). Functional Surgery for Malignant Subungual Tumors: A Case Series and Literature Review A retrospective study from Mexico and Brazil tracked patients who had digit-preserving surgery, including some with invasive melanomas: among seven patients with invasive disease who underwent functional surgery, five remained alive and disease-free over follow-up periods of up to 27 months.20Skin Appendage Disorders. Non-Amputative Digit Preservation Surgery versus Amputation in Nail Unit Melanoma: A Retrospective Study in Mexico and Brazil Amputation remains the recommended approach for very thick tumors (over 4 mm), tumors involving bone, and recurrent cancers.
Practical Steps if You Notice a Dark Line
You do not need to rush to the emergency room, but you should not ignore it either. A few things to do right away:
- Photograph it: Take a clear, well-lit photo with something for scale (a coin or ruler). Date it. Repeat every few weeks. Having a visual record of whether the band is stable or changing is the single most useful thing you can bring to a dermatologist.
- Think about timing: Did this appear after an injury? After starting a medication? Has it been there as long as you can remember? These details help your doctor narrow things down quickly.
- Check your other nails: Multiple nails with similar streaks point strongly toward a benign systemic cause like ethnic melanonychia or a drug reaction. A single affected nail demands closer scrutiny.
- Look at the surrounding skin: Pigment spilling onto the cuticle or the skin beside the nail is the feature that most reliably raises concern, even though benign mimics exist.
If the band is new, solitary, wider than about 3 mm, has irregular color or borders, or is accompanied by nail distortion or bleeding, see a dermatologist. If your doctor suspects something beyond a benign cause, dermoscopy and possibly a biopsy will clarify the picture. The entire point of awareness is not to panic over every dark nail, since the vast majority are harmless, but to know that the rare dangerous exception looks so similar to benign conditions that only professional evaluation can tell them apart.
Why Toenails Get Overlooked
One factor that makes subungual melanoma of the toenail especially treacherous is simple visibility. People look at their fingernails every day but may go weeks without closely inspecting their toes, especially during cooler months when feet stay covered. Nail polish further obscures changes. The 85% misdiagnosis rate cited in the literature is not just about clinical difficulty; it is also about patients presenting late because they did not notice anything was wrong, or assumed a dark spot was just a bruise from exercise.11PubMed Central. A Misdiagnosed Case of Malignant Melanoma in an Infected Nail: A Case Report The great toe is the most commonly affected digit for subungual melanoma, possibly because it absorbs more trauma, which is also the toe most people would dismiss as “just a runner’s nail” or attribute to tight shoes. Making a habit of actually looking at your bare toenails every few weeks, the same way you might check a mole on your arm, is a surprisingly effective screening measure for something most people never think to do.