OBJECTIVE: To evaluate the contribution of anovulation and luteal phase defects to lactational infertility. DESIGN: Prospective longitudinal follow-up. SETTING: Outpatient clinic. SUBJECTS: Forty-nine women fully nursing and amenorrheic on day 75 postpartum and 25 cycling, interval non-nursing women. INTERVENTIONS: Plasma prolactin, luteinizing hormone, estradiol (E2), and progesterone (P) levels twice a week up to the second postpartum menses. MAIN OUTCOME MEASURES: Ovulation rate and endocrine profile of the menstrual cycles. RESULTS: Ovulation rates were 37% and 97% at 6 and 12 months postpartum; 67% of ovulations occurred in amenorrhea. The luteal phase was shorter, and E2 and P levels were lower in lactating women than in non-nursing women. These parameters were closer to normal in the second cycle than the first, in spite of active nursing. The risk of ovulation and pregnancy in amenorrhea was 27.7% and 0.9% at month 6 postpartum. After the first menses, these risks were 93% and 7%, respectively. CONCLUSION: The abnormal endocrine profile of the first luteal phase offers effective protection to women who ovulate during lactational amenorrhea within the first 6 months after delivery. Later luteal phases are improved and women are at risk of pregnancy.