OBJECTIVE: Our purpose was to determine whether infertile patients who have endometriosis show luteal phase defects. STUDY DESIGN: The luteal function in 24 infertile patients who had endometriosis was compared with the luteal function in 20 patients who had unexplained infertility and did not have endometriosis (control). In both groups serum luteinizing hormone, follicle-stimulating hormone, progesterone, and estradiol were assayed every day throughout the menstrual cycle. Endometrial biopsy specimens were obtained from eight patients of the endometriosis group for histologic dating of the endometrium. RESULTS: No significant differences in progesterone levels were observed between these two groups during the mid and late luteal phase. Seven of the eight patients who underwent histologic dating showed a luteal phase pattern, whereas only one patient was out of phase. CONCLUSION: Infertile patients who have endometriosis do not always have luteal phase defects.