Pre-menopausal (14) and post-menopausal (55) patients with advanced breast cancer were treated with danazol (Danol) for periods ranging between 3 days and 30 months. Of these, 10 patients (14.9%) responded to treatment for 3-19 months (mean 10 months); a further six patients (9%) showed stabilisation of disease; 45 patients showed clear progression of disease following treatment; and eight patients were unassessable. Side-effects occurred in 17 of 69 patients (25%) and necessitated a reduction in therapy in eight cases. It is concluded that danazol is moderately well tolerated and is an effective agent in patients with advanced breast cancer, but the response rate is inferior to that of other agents in current use, such as tamoxifen or aminoglutethimide.