A black or dark brown line running lengthwise on your nail is usually caused by one of a few common, benign conditions. The most frequent cause is a small bleed under the nail (accounting for about 29% of cases), followed by a mole in the nail matrix (about 22%) and trauma (about 15%). In rare cases, a dark nail streak can signal melanoma, so it’s worth understanding what to look for.
The Most Common Cause: Bleeding Under the Nail
A bleed beneath the nail, called a subungual hematoma, is the single most common reason for brown-black nail discoloration. It happens when tiny blood vessels under the nail rupture, either from a single hard hit (like slamming your finger in a door) or from repeated low-grade pressure over time. An acute injury typically produces a deep red or purple spot that doesn’t extend all the way to the tip of your nail. Chronic, repeated microtrauma, on the other hand, can create a red-brown streak that looks remarkably like a pigmented line running the length of the nail.
A related pattern is a splinter hemorrhage: a thin, dark line that looks like a tiny splinter embedded under the nail. These form when capillaries that run lengthwise beneath the nail plate burst. When they appear near the tip of the nail, trauma is almost always the culprit. Nail biting, using a cane, or repetitive hand work can all trigger them. If multiple splinter hemorrhages show up closer to the base of several nails at once, that pattern is more concerning and can be linked to systemic conditions like a heart valve infection.
Normal Pigmentation in Darker Skin Tones
If you have a darker complexion, vertical brown or black bands on one or more nails may simply be part of your normal pigmentation. This is extremely common in people with Black, Asian, Hispanic, or Middle Eastern backgrounds. The pigment-producing cells in the nail matrix are more active in darker skin types, and these bands often appear on multiple nails. They tend to be stable in color and width over time, and no treatment is needed.
Moles in the Nail Matrix
A mole (nevus) located in the nail matrix, the tissue under the base of your nail where the nail grows from, can produce a light brown to black band that runs from the base to the tip. These bands are typically 3 to 5 mm wide and most often show up on the thumb. Nail moles account for about 12% of dark nail streaks in adults and nearly half of them in children. Most are benign, but because they involve pigment-producing cells, a dermatologist may want to monitor them over time for any changes.
Infections and Other Triggers
Fungal infections can occasionally produce dark pigmentation on the nail rather than the typical yellow-white thickening most people associate with nail fungus. Certain bacteria, particularly Pseudomonas (which more commonly turns nails green), can also cause dark discoloration. If the line appeared alongside other nail changes like crumbling, thickening, or a foul smell, infection is a possibility worth exploring.
Some medications cause dark nail bands as a side effect. Chemotherapy drugs are the most common offenders, but other systemic treatments can trigger it too. The good news is that drug-related nail changes are generally painless and reversible within a few months of stopping the medication. Pregnancy can also cause temporary dark bands across several nails at once, driven by hormonal shifts that activate pigment cells.
Inflammatory skin conditions like psoriasis and lichen planus, when they affect the nail area, can stimulate extra pigment production and create dark streaks as well.
When a Dark Line Could Be Melanoma
Melanoma under the nail is rare, representing roughly 1 to 3.5% of all melanomas in lighter-skinned populations. But it does happen, and because it’s hidden under the nail, it’s often caught late. Dermatologists use a set of warning signs to evaluate dark nail streaks:
- Age and ancestry: Risk is higher in adults between 20 and 90, particularly in people of Asian or African American descent.
- Band appearance: A brown or black band wider than 3 mm is more suspicious, especially if it has irregular color or blurred borders.
- Change over time: A band that is rapidly widening, darkening, or developing irregular edges deserves prompt attention.
- Digit involved: The thumb of the dominant hand is the most common location for nail melanoma.
- Pigment spreading to surrounding skin: This is called a Hutchinson sign, where dark pigment extends from under the nail onto the cuticle or the skin around the nail. First described in 1886, it typically appears in more advanced stages of nail melanoma and is considered a serious red flag.
- Family history: A personal or family history of melanoma or atypical moles raises risk.
No single feature on this list confirms cancer. But if your nail streak matches several of these patterns, especially if it’s new, changing, or spreading onto the surrounding skin, it warrants evaluation sooner rather than later.
What Happens During Evaluation
A dermatologist will first examine the streak visually and with a dermatoscope, a handheld magnifying device with a light that reveals pigment patterns invisible to the naked eye. Dermoscopy can even detect a “micro-Hutchinson sign,” pigment spreading into the cuticle that’s too subtle to see without magnification.
If the streak looks suspicious, the next step is a biopsy of the nail matrix. There are two main approaches. A punch biopsy removes a small cylinder of tissue (up to 3 mm from the matrix). A shave biopsy, more accurately called a tangential excision, is the preferred technique for pigmented streaks because it samples the exact layer where pigment cells sit. Stitches from the area around the base of the nail typically come out after about a week, and normal nail regrowth begins around four weeks after the procedure. When performed properly, these biopsies heal without scarring or permanent nail deformity.
How to Tell What You’re Dealing With
A few practical clues can help you sort harmless from concerning. If you recently injured the finger, the line appeared suddenly, or you can see a reddish-purple hue rather than true brown-black, a bruise is the most likely explanation. It will grow out with the nail over several months. If you have dark skin and similar bands on multiple nails that haven’t changed in years, normal ethnic pigmentation is overwhelmingly likely.
A streak that appeared on a single nail without any injury, is wider than 3 mm, has darkened or widened over weeks to months, or shows pigment bleeding into the skin around the nail is the pattern that calls for professional evaluation. The same applies to a new dark band on the thumb of your dominant hand, particularly if you’re over 40.