The ABCDEs of skin cancer are five visual features that help you spot a potentially dangerous mole: Asymmetry, Border, Color, Diameter, and Evolving. Developed specifically for melanoma detection, this checklist gives you a simple framework for evaluating any spot on your skin during a self-exam. Here’s what each letter means and how to use them.
A: Asymmetry
Draw an imaginary line through the center of a mole. If the shape of one half doesn’t match the other, that’s asymmetry. Normal moles tend to be roughly round or oval with a balanced shape. A melanoma is more likely to look lopsided, with one side noticeably larger, raised, or differently shaped than the other.
B: Border
Healthy moles typically have smooth, well-defined edges. A worrisome mole often has borders that are ragged, notched, or blurred. In some cases the pigment appears to spread or fade into the surrounding skin rather than stopping at a clean edge. Think of it as the difference between a circle drawn with a ruler versus one drawn freehand with a shaky hand.
C: Color
Most normal moles are a single, uniform shade of brown. Melanomas frequently contain multiple colors within the same spot: shades of black, brown, and tan mixed together, sometimes alongside areas of white, gray, red, pink, or blue. The more colors present, the more suspicious the lesion. Blue or gray tones within a mole are particularly notable, as are white patches that look like scarring. Five or six different colors in a single spot is a strong warning sign.
D: Diameter
The traditional threshold is 6 millimeters, roughly the size of a pencil eraser. Moles wider than this deserve closer attention. That said, melanomas can absolutely be smaller than 6 millimeters, especially in their early stages. Diameter is best used alongside the other criteria rather than as a standalone rule. A tiny mole that’s asymmetric, multicolored, and changing is more concerning than a large, stable, uniform one.
E: Evolving
This is often the most important letter. Any mole that has changed over the past few weeks or months qualifies as “evolving.” That includes changes in size, shape, color, or texture. It also includes new symptoms: itching, pain, bleeding, or crusting that wasn’t there before. A mole that suddenly looks different from how it looked a month ago warrants attention, even if it doesn’t clearly meet any of the other ABCDE criteria.
The Ugly Duckling Sign
The ABCDE framework evaluates each mole on its own. The “ugly duckling” sign takes a different approach: it asks whether one mole stands out from all the others on your body. Most people’s moles share a general family resemblance in color, size, and shape. A mole that looks nothing like the rest, even if it doesn’t obviously fail the ABCDE test, may still be suspicious.
Researchers in France found that dermatologists using the ugly duckling method were able to identify melanomas just as accurately as those applying detailed diagnostic criteria to each mole individually. The ugly duckling approach also reduced unnecessary biopsies. In practice, it’s worth using both methods together: check each concerning mole against the ABCDEs, but also step back and ask whether any single spot looks like the odd one out.
What the ABCDEs Don’t Catch
The ABCDE rules were designed for the most common type of melanoma, which tends to spread outward across the skin surface. Nodular melanoma, a faster-growing variant, often doesn’t follow these patterns. Instead of appearing as a flat, irregularly shaped spot, nodular melanomas typically show up as raised, firm bumps that grow rapidly over days to weeks. They may be uniform in color, symmetrical, and smaller than 6 millimeters.
For this type, dermatologists use a separate set of criteria: Elevated, Firm, and Growing. Any new bump on your skin that feels firm to the touch and is visibly growing over a short period is worth having checked, even if it looks nothing like the textbook melanoma described by the ABCDEs.
The ABCDE system also applies specifically to melanoma. Other common skin cancers, like basal cell carcinoma and squamous cell carcinoma, often look quite different. They may appear as pearly or waxy bumps, flat reddish patches, or open sores that don’t heal. A spot that hurts, bleeds, itches persistently, or simply won’t go away is worth professional evaluation regardless of whether it matches the ABCDE pattern.
How to Do a Self-Exam
Check your skin about once a month, ideally after a bath or shower when your skin is clean and visible. You’ll need good lighting and two mirrors (a full-length and a handheld) to see hard-to-reach areas.
The spots people most commonly miss are the ones that are hardest to see: your back, the backs of your legs, the soles of your feet, between your toes, your scalp, the back of your neck, and the skin under your breasts if applicable. Use mirrors to inspect your back and buttocks. Sit down to check the bottoms of your feet. Part your hair in sections or use a blow dryer to expose your scalp.
You’re not trying to diagnose anything during a self-exam. You’re building a mental map of what your skin normally looks like so you can notice when something changes. A spot that follows any of the ABCDE rules, that looks dramatically different from your other moles, or that hurts, bleeds, itches, or simply worries you is reason enough to see a dermatologist. If a spot looks concerning, a dermatologist can numb the area and take a small tissue sample right in the office to determine whether it’s harmful.