A skin condition that causes dark, velvety patches in body folds and creases.

Acanthosis nigricans is more likely in people who are obese or have diabetes. More rarely, it can be a warning sign of a cancerous tumor in an internal organ, such as the stomach or liver.

It often affects the armpits, groin, and neck. The affected skin can thicken and may smell bad.

Treating the underlying condition, such as obesity or a tumor, may restore the skin.