To elucidate the roles of prostaglandins in peritoneal fluid and sex steroids in patients with endometriosis (N=29), tubal disorders (N=15), and unexplained infertility (N=13), assays were performed using 6-keto-prostaglandin F1α (6-keto-PGF1α) (a metabolite of prostacyclin), thromboxane B2 (a metabolite of thromboxane A2), estradiol, and progesterone. Women with normal pelvic anatomy (N=25) served as controls. Peritoneal fluid 6-keto-PGF1α concentrations in patients with endometriosis (742 ± 104 pg/ml, mean ± SE), tubal disorders (987 ± 211 pg/ml), and unexplained infertility (1659 ± 770 pg/ml) were higher than those in the control women (515 ± 77 pg/ml). The thromboxane B2 levels in the peritoneal fluid in endometriosis (554 ± 73 pg/ml), tubal disorders (614 ± 107 pg/ml), and unexplained infertility (668 ± 161 pg/ml) were higher than the levels in the control subjects (333 ± 23 pg/ml). There was no relationship between 6-keto-PGF1α/thromboxane B2 in peritoneal fluid and day of menstrual cycle. The concentrations of estradiol and progesterone were normal in all patient groups and were not related to the 6-keto-PGF1α and thromboxane B2 levels. The authors suggest that these prostanoids, which may contribute to infertility, may originate mainly from the peritoneum as a result of irritation by endometriotic implants, tubal adhesions, and scarring.