Acanthosis nigricans is a skin condition that results in darkened skin and velvety growths that look like warts. These skin changes can occur under the arm, on the back of the neck, over finger and toe joints, or in the groin and genital area.

In adults, acanthosis nigricans may be a sign of disorders of the hormone system (endocrine system). It can occur in people who are very overweight (obese). It may be a sign of lung cancer or of a cancer of the digestive system. It can occur in people who have type 2 diabetes and in people who are resistant to insulin but have not yet developed type 2 diabetes. Acanthosis nigricans can occur in women who have polycystic ovary syndrome (PCOS).