The purpose of the investigation was to study various aspects of danazol treatment in patients with fibrocystic breast disease and pronounced mastodynia. To qualify for inclusion, the patients in this study had to have a mammographically confirmed prominent glandular structure and/or severe cystic breast disease, associated with pronounced cyclical mastodynia lasting at least one week per menstrual cycle for more than 6 months. They also had to be premenopausal and not undergoing hormonal therapy. Of 109 patients with a mean age of 40 +/- 6.2 (SD) years, who completed 6 months' treatment with danazol, 65% had a pre-treatment history of more than 5 years. According to detailed mammographic characterization, only 18% of the patients had no visible cysts. Of the 82% with visible cysts, half presented with both small (less than 1 cm) and large cysts. The patients were treated in two consecutive groups, 55 patients receiving 400 mg a day and 54 patients 200 mg a day. The therapeutic response was similar following both dosages. Mastodynia responded rapidly and total elimination was noted in about 90% of cases. A marked decrease in prominence of palpable structure was observed in virtually all patients. Mammographically, a decrease in the amount of glandular tissue was observed. These changes during treatment were statistically highly significant (p less than 0.001), irrespective of dose or category of patient. Non-cystic nodularities gradually decreased in 85% and resolved completely in 58% of the cases, but the degree of resolution in the groups of patients with no or only small (less than 1 cm) visible cysts was significantly greater (p less than 0.02 or less) than in the group which also had large cysts and which included the most severe and intractable cases. The mammographical visualization of cysts, ductal system, and fibrous tissue increased initially due to the marked regression of obscuring glandular tissue. Thereafter, a decrease in the number and spread of small cysts was observed in a significant proportion of patients and in some cases a reduction in duct diameter could be demonstrated by means of galactography. Nipple discharge also decreased. The extent of fibrosis appeared to be unaffected by therapy. In the 46 patients with large cysts and, in most cases, a documented history of repeated cyst formation, danazol treatment was found to arrest the development of new cysts and to prevent recurrence for a considerable time thereafter.(ABSTRACT TRUNCATED AT 400 WORDS)