Urinary levels of testosterone, 5 alpha-androstanediol, 17-hydroxycorticosteroids, pregnanediol, and circulating levels of testosterone, 17 beta-estradiol, dehydroepiandrosterone-sulfate, prolactin, luteinizing hormone, follicle-stimulating hormone and sex hormone-binding globulin, were measured in 10 male patients with breast cancer and in a suitable group of healthy controls. No difference, either in blood or in urine, was observed between the two groups in the hormonal levels. The lack of abnormalities in peripherally detectable hormones suggests that the well recognized hormone dependency of male breast cancer may be due to some endocrine imbalance in the central (diencephalic) regulation of the sex steroids pathway. Alternatively, abnormal response of breast tissue to normal hormonal stimuli might be hypothesized in these patients.