Fibrocystic disease (chronic cystic mastitis) is a common disorder of the adult female breast. It is rare after cessation of ovarian function, either natural or induced. Its association with estrogens is inferential. In postmenopausal women it only occurs when replacement estrogen therapy is in use. Its main clinical significance relates to the need to differentiate irregular breast tissue from cancer. Patients afflicted with this disorder are often frustrated by the repeated biopsies that may be recommended.