The results of exploration of the corpus luteum in 125 patients with benign breast disease are analysed. In 109 patients, plasma oestradiol and progesterone were measured at various times during the hyperthermic phase of the menstrual cycle. The results obtained were compared with those obtained in normal women explored during the same phase of the cycle. The average daily values for plasma progesterone obtained in this group of 109 patients was significantly lower than that found in the normal women, whilst there was no change in oestradiol levels. In addition, 16 patients were studied daily throughout the period of hyperthermia. In 8 cases, there the hyperoestrogenism, with or without progesterone insifficiency of the corpus luteum. In all, secretory imbalance of the corpus luteum with a tendency to relative or absolute hyperoestrogenism was a constant finding in women with benign breast disease.