Atypical breast hyperplasia is a precancerous condition that affects the cells in the breast. Atypical hyperplasia describes the accumulation of abnormal cells in the breast ducts.

Breast hyperplasia is a benign breast condition that usually causes no symptoms such as pain or lumps and is often discovered incidentally. It occurs when certain cells lining the surface of the ducts or lobules of the mammary gland proliferate. Tubular hyperplasia can be either typical or atypical ductal hyperplasia depending on the histological appearance on microscopic examination.