Danazol in a dose of 400 mg daily was administrated to 40 patients with chronic cystic mastitis and resulted in a marded improvement in both objective and subjective symptoms in 87.5% of the patients studied. Three patients showed partial relief of symptoms and 1 patient showed no improvement after 1 month of treatment at which time she withdrew from the study. One patient had a worsening of her condition and was withdrawn from the study after 3.5 months of therapy. Nineteen of the 40 patients in the study had a secondary diagnosis of pelvic endometriosis confirmed histologically, and all these patients showed a marked improvement of symptoms during the Danazol treatment. Patients developed amenorrhea after 3 to 4 months of Danazol therapy, and symptoms such as dysmenorrhea, premenstrual pelvic pain, and tension abated at the same time. A mild, but well-tolerated weight gain was the major side effect of Danazol administration. No significant changes in the levels of plasma E1, E2, FSH, LH, or progesterone could be demonstrated in specimens drawn from 11 patients before and during the course of Danazol administration.