A phase II study with cyproterone acetate (CPA) was done as the primary treatment in female breast cancer patients. Twenty-three patients, mean age 64 years, range 52-75 years, were entered and treated with CPA 400 mg daily. Twenty patients were evaluable and responses were sparse. There was one partial and one complete remission, 17 patients were stable and one patient progressed within 3 months. Side-effects were frequent: five patients complained of nausea, three had severe weight loss, one suffered from depression and seven showed disturbed liver function tests. Six patients had to stop treatment for side-effects, while two other patients were taken off treatment because they developed an acute necrotizing hepatitis. The hepatitis recovered after drug withdrawal in both patients. The serum levels of CPA, cortisol, androstenedione, DHAS, LH, FSH and prolactin were measured during CPA treatment. The levels of cortisol and androstenedione did not change, while LH, FSH and DHAS were suppressed. The DHAS showed an inverse relation to serum CPA concentrations. The prolactin levels rose uniformly. The therapeutic effect of CPA in postmenopausal patients with advanced breast cancer is disappointing, and inferior to that of other progestins. Side-effects are frequent, possibly as a result of the high dosage used in this study. The hormonal changes are different from those of other progestins, which may explain the different efficacies.