OBJECTIVE: To investigate whether luteal and endometrial abnormalities occur more frequently in an infertile population and thus contribute to infertility. DESIGN: Prospective controlled clinical study. SETTING: Outpatient clinic in an academic research institution. PARTICIPANTS: Thirty-three fertile controls and 31 infertile women without ovulatory disorders, tubal disease, or male factors. INTERVENTIONS: All women underwent an endometrial biopsy 9 days after the LH surge followed by an IM injection of 5,000 IU hCG. Blood samples were drawn immediately before hCG administration for serum P and placental protein 14 (PP14) measurements, at 6 hours after hCG stimulation for serum P concentrations, and on day 5 after hCG administration for serum PP14 levels. MAIN OUTCOME MEASURES: Histologic dating of the endometrium and serum P and PP14 measurements. RESULTS: Abnormal endometrial biopsies occurred more frequently in infertile (43%) than in fertile women (9%). Except for one case, these specimens were not associated with low hCG-stimulated P levels. Serum PP14 measurements varied widely and did not discriminate subjects with abnormal endometrial development. CONCLUSIONS: Disruption of endometrial maturation without a concomitant defect of the corpus luteum occurs more frequently in an infertile population and thus may contribute to infertility.