Serum prolactin values on patients with breast cancer were measured by double antibody radioimmunoassay. In postmenopausal normal and women with mastopathy disease serum prolactin values were higher than premenopausals, and in premenopausal breast cancer patients they were higher than postmenopausals. But these results were within normal limits. Serum prolactin values of breast cancer patients after ablative hormone therapy also were within normal limits.