One hundred eighty-four patients with benign breast disease (BBD) were studied and compared with 50 normal women. All of the women had ovulatory cycles according to a biphasic basal body temperature and a plasma prolactin in the normal range. Their corpus luteum function was evaluated by way of plasma progesterone (P) and estradiol (E,) determinations at days 5, 7, and 9 of the hyperthermic phase. In the 184 patients, plasma P over plasma E, ratio during the luteal phase was found significantly lower than in normal women. When the patients were grouped according to type of breast lesions, it appeared that plasma P was constantly lower in all groups than in the normal women, while plasma E2 was either normal or elevated in the groups of patients with adenosis tumors and increased odularity of both breasts. From these results it may be postulated that an imbalance in the secretion of E2 and P by the corpus luteum is a constant finding in women with benign breast disease.