Serum testosterone, progesterone, luteinizing hormone, prolactin, and sex hormone-binding globulin levels were measured in 55 normal controls, in 31 patients with hyperplastic alterations of breast epithelium, and in 23 patients with breast cancer. All patients and controls were premenopausal, and they were comparable for age, weight, and body surface. In the controls, the mean level of testosterone [0.47 +/- 0.16 (S.D.) ng/ml] was lower and the mean level of progesterone (17.63 +/- 8.11 ng/ml) was higher than in breast cancer patients [testosterone level, 0.62 +/- 0.22 ng/ml (p less than 0.005); progesterone level, 11.4 +/- 8.0 ng/ml (p less than 0.005)] and in patients with breast epithelial hyperplasia [testosterone level, 0.55 +/- 0.2 ng/ml (p less than or equal to 0.05); progesterone level, 13.9 +/- 8.6 ng/ml (p less than 0.05)]. No difference was found in the mean circulating levels of luteinizing hormone, prolactin, or sex hormone-binding globulin between controls and patients. These results confirm previous findings of increased urinary testosterone excretion in women with anovulatory menstrual cycles and epithelial hyperplasia or cancer of the breast and strongly support the hypothesis that androgens play an important role in both induction and development of breast cancer.