The corpus luteum function of 109 patients with benign breast disease was appreciated by way of plasma progesterone and estradiol determinations during the luteal phase. These patients had ovulatory cycles according to a biphasic basal body temperature curve; blood samples for plasma progesterone and estradiol estimation were collected between the first and the last day of the thermal plateau following the thermal nadir. Results obtained were compared to those observed in 50 normal ovulatory women. In the patients' group, the mean daily levels for progesterone ranged from 3.5 +/- 0.4 (SE) ng/ml to 8.1 +/- 3.8 (SE) ng/ml according to day of blood collection. These values are significantly different from the corresponding daily values observed in normal women. No significant difference was observed concerning plasma estradiol between patients and normal women. These findings indicate that women with benign breast disease have an inadequate corpus luteum function which may be secondary to an ovulation disorder. Pathophysiological implications resulting from this observation are discussed.