Different serum hormones were studied in patients with benign breast diseases and breast carcinoma in respect of different phases of the menstrual cycle, as well as in postmenopausal women. In premenopausal breast carcinoma subjects 10% showed elevated serum estradiol alone, 7% showed elevated serum prolactin alone, and 12% subjects exhibited elevated levels of both serum estradiol and prolactin. Similarly, in postmenopausal breast carcinoma subjects 12% showed elevated serum estradiol alone, 10% showed elevated serum prolactin alone, and 22% exhibited elevated level of both serum estradiol and prolactin. On the other hand, in patients with benign breast disease only 5 showed an elevated level of prolactin alone. More than 50% of premenopausal women with carcinoma of the breast had low level of serum progesterone during the luteal phase as compared to normal subjects. No variations in serum follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were evident between normal subjects and women with breast carcinoma or benign breast disease. The increased level of serum estradiol and prolactin may be useful in the diagnosis of human breast cancer.