How to Tell If a Mole Is Cancerous: Warning Signs

Most moles are harmless, but knowing how to spot a cancerous mole could save your life. Melanoma, the most serious type of skin cancer, is highly treatable when caught early—with a five-year survival rate exceeding 99% for localized cases. The key is recognizing warning signs before cancer spreads.

Learning to identify suspicious moles through regular skin checks helps detect melanoma in its earliest, most treatable stages. Here's what to look for and when to seek professional evaluation.

What Normal Moles Look Like

Healthy moles share common characteristics that distinguish them from potentially cancerous growths:

• Symmetrical shape (both halves match if you draw a line through the middle)

• Smooth, even borders

• Uniform color (single shade of brown, tan, or flesh-colored)

• Small size (typically under 6 millimeters, about the size of a pencil eraser)

• Stable appearance over time

Most people develop between 10 and 40 moles throughout their lifetime, with most appearing before age 30. Normal moles on your body tend to resemble one another in color and size.

The ABCDE Rule for Melanoma Detection

Dermatologists use the ABCDE rule to help identify potentially cancerous moles. This simple method flags warning signs that require professional evaluation:

A - Asymmetry

One half of the mole doesn't match the other half in shape or appearance. Melanomas are typically asymmetrical, while normal moles are round or oval and symmetrical.

B - Border

The edges are irregular, blurred, notched, or poorly defined. Healthy moles have smooth, even borders, while melanoma borders often appear ragged or scalloped.

C - Color

The mole has multiple colors or shades. Normal moles are usually a single shade of brown. Melanomas may display different shades of brown, tan, or black, and as they grow, may include red, white, or blue areas.

D - Diameter

The spot is larger than 6 millimeters across (about the size of a pencil eraser). While melanomas can be smaller, any mole larger than this size warrants attention.

E - Evolving

The mole is changing in size, shape, color, or elevation, or developing new symptoms like bleeding, itching, or crusting. Evolution is considered the most important warning sign. Any noticeable change in an existing mole or the appearance of a new mole after age 30 should be evaluated.

The Ugly Duckling Sign

Beyond the ABCDE rule, dermatologists recommend watching for the "ugly duckling" sign—a mole that looks noticeably different from your other moles. Since most normal moles on your body resemble one another in pattern, size, and color, melanomas often stand out as the odd one.

Research shows the ugly duckling sign significantly improves melanoma detection accuracy. If one mole looks different—whether larger, smaller, darker, lighter, or just unusual compared to others—have it checked by a dermatologist.

When to See a Dermatologist

Schedule a skin exam if you notice:

• Any mole that meets one or more ABCDE criteria

• A mole that looks different from your other moles

• A new mole appearing after age 30

• Any mole that bleeds, itches, or becomes painful

• A mole with a crusty or scaly surface

Consider annual full-body skin exams if you have a personal or family history of melanoma, fair skin, many moles (more than 50), a history of sunburns, or are over age 60.

Performing Regular Skin Self-Exams

Monthly self-exams help you become familiar with your skin and notice changes early. Check your entire body in a well-lit room using a full-length mirror and hand mirror for hard-to-see areas. Don't forget to examine your scalp, between fingers and toes, soles of feet, and under nails.

Take photos of any moles you're monitoring so you can track changes over time. While about 70-80% of melanomas develop on seemingly normal skin, 20-30% arise from existing moles, making regular monitoring essential.

Frequently Asked Questions

Can a normal-looking mole become cancerous?

Yes, though most melanomas appear as new spots. About 20-30% of melanomas develop from existing moles, which is why monitoring changes in your moles is important.

How quickly can a mole become cancerous?

Melanoma can develop rapidly, sometimes within weeks to months. This is why the "evolving" part of the ABCDE rule is so critical—any noticeable change warrants immediate evaluation.

Are raised moles more likely to be cancerous?

Not necessarily. Both flat and raised moles can be normal or cancerous. What matters more is whether the mole shows ABCDE warning signs or changes over time.

The Bottom Line

Early detection of melanoma dramatically improves survival rates. By learning the ABCDE rule, watching for ugly duckling moles, and performing monthly self-exams, you can catch suspicious changes early. When in doubt, always have a dermatologist evaluate any concerning moles. A simple skin check could save your life.