Subungual Melanoma: How to Spot the Nail Cancer Hiding in a Dark Stripe

You noticed a dark vertical stripe under your fingernail or toenail, and now you can't stop thinking about it. Most of these bands are completely harmless, but you've probably read the word "melanoma" and want a straight answer. Here is what that stripe could mean, how to tell the worrying kind from the everyday kind, and exactly when it's time to get it checked.

Subungual melanoma is a rare but serious skin cancer that grows in the tissue beneath the nail. It's very treatable when caught early, which is why knowing the warning signs matters so much.

What is subungual melanoma?

Subungual melanoma is a melanoma that starts in the nail matrix, the area beneath the base of the nail where new nail grows. It is a type of acral lentiginous melanoma, an aggressive form of skin cancer that tends to appear on the palms, soles, and nails rather than sun-exposed skin. Because it hides under the nail, it is easy to miss and easy to mistake for a bruise or fungal problem.

It most often develops in the big toe, thumb, or index finger. In white and Western populations it accounts for only about 2% of all melanomas of the skin, but it makes up a far larger share of melanomas in people with darker skin: up to 75% of melanomas in Africans, roughly 25% in Chinese of Hong Kong, and about 10% in Japanese. Peak incidence falls in the 5th to 7th decades of life.

What does subungual melanoma look like?

The most common sign is a brown or black vertical line running from the base of the nail to the tip. In about 65% of cases, nail melanoma first appears as a single dark pigmented band, often wider than 3 mm, that gets wider toward the base of the nail and has irregular or blurred borders.

One important catch: up to a third (about 33%) of subungual melanomas are amelanotic, meaning they show no dark pigment at all. These can look like a stubborn sore, a lump, or a nail that won't heal, which is part of why diagnosis is so often delayed. Beyond a dark band, watch for:

• A pigmented band that keeps widening or has uneven color
• A non-healing wound or sore under or around the nail
• A nodule, lump, or split in the nail plate
• Bleeding, nail deformity, or the nail lifting or falling off
• Dark pigment spreading onto the surrounding skin or cuticle (the Hutchinson sign)

The ABCDEF rule for nail melanoma

Dermatologists use a memory aid called the ABCDEF rule to flag nail changes that need evaluation. It is a guide for clinical detection, not a self-diagnosis tool, but it tells you what doctors look for:

• A — Age (peak in the 5th to 7th decades) and higher-risk backgrounds, including African American, Asian, and Native American heritage
• B — Band that is Brown to black, Breadth of 3 mm or more, with Blurred or irregular borders
• C — Change in the band over time, or no change despite treatment
• D — Digit most often involved: the thumb, great toe, or index finger
• E — Extension of pigment onto the nailfold or surrounding skin (the Hutchinson sign)
• F — Family or personal history of melanoma or dysplastic (atypical) moles

Subungual melanoma vs. a harmless dark nail stripe

A single dark vertical band in the nail is called longitudinal melanonychia, and most of the time it is benign, especially in people with naturally darker skin who often have several such bands. The challenge is that melanoma can look nearly identical at first.

Reassuring features tend to include a thin, even, stable band with consistent color, sometimes appearing in more than one nail. More concerning features include a single band that is wide, getting wider, irregularly colored, changing, or accompanied by pigment spreading onto the skin. The Hutchinson sign (pigment extending onto the nailfold) has long been treated as a strong clue for melanoma, though it is not perfectly reliable on its own, because a harmless pseudo-Hutchinson sign also exists. Only a doctor can reliably tell these apart, often with dermoscopy and, when needed, a biopsy.

How is it diagnosed and treated?

Histopathology, examining a tissue sample under a microscope, remains the gold standard for diagnosing a suspicious nail band. A dermatologist may first use dermoscopy to look at the nail more closely, then take a biopsy of the nail matrix if the pattern is concerning. This is the only way to know for certain whether a band is melanoma.

Treatment depends on how deep and advanced the melanoma is, and is led by a dermatologist or surgical specialist, frequently with surgery to remove the cancer. The takeaway for you is timing: prognosis is closely tied to how early it's found. Average diagnostic delay for subungual melanoma is about two years, and that delay is a major reason outcomes can be poor, so getting a changing nail checked promptly genuinely matters.

When to see a doctor

See a dermatologist promptly if you have a new or changing dark band in a single nail, a nail sore or lump that won't heal, pigment spreading onto the skin around the nail, or any of the ABCDEF warning signs above. Do not wait for it to hurt, since many of these lesions are painless.

The reason to act is straightforward. Overall five-year survival for subungual melanoma ranges widely, from about 15% to 59%, and reaches roughly 95% when the cancer is caught at an early stage but can fall to as low as 15% once it has spread. Toe lesions tend to carry a worse outlook than finger lesions, with around 40% five-year survival for invasive subungual toe melanoma versus about 72% for finger lesions. Early evaluation is the single most powerful thing in your control. If you're unsure whether a nail change needs a look, a clinician-overseen skin check, including tools like Nolla's skin scans, can help you decide when to get seen in person.