Serum levels of progesterone and estradiol at early, middle, and late luteal phases were compared in nonfertile cycles in which only ovulation occurred to those in cycles in which ovulation was verified by conception. Two groups of patients were studied: 33 normal ovulatory women (12 of whom conceived) and 28 women who underwent induction of ovulation (8 of whom conceived). In each group, mean midluteal levels of progesterone and estradiol were essentially similar in nonfertile and fertile cycles. In fertile cycles, the late luteal levels of both steroids were higher than their respective midluteal values, whereas the late luteal values dropped in the nonfertile cycles. The late luteal rise in estradiol was found to be a more sensitive indicator of pregnancy than that of progesterone. The progesterone/estradiol ratio was stable during the luteal phase of fertile and nonfertile cycles. The ratio in spontaneous cycles was 34.4 +/- 4.7 and only 17.9 +/- 4.3 in induced cycles. This marked decrease suggests an impairment of steroid metabolism in women who require induction of ovulation.