Androstenedione, estrone, estradiol-17.beta., estriol and testosterone concentrations were measured by radioimmunoassay in plasma of 17 human males affected by breast cancer. The mean values of estrone, estradiol-17.beta. and estriol in male breast cancer were significantly higher than in normal controls of comparable age. Androstenedione and testosterone were in the normal range. Orchidectomy, performed in 3 subjects, decreased the levels of estradiol-17.beta., but to a lesser extent, those of estrone and estriol.