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, or in the groin and genital area.

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