An anatomic basis for the infertility associated with endometriosis is often lacking. The present study measured peritoneal fluid levels of two of the stable products of prostaglandin endoperoxides (thromboxane B2 and 6-keto-prostaglandin F1 alpha) in patients with and without endometriosis. Both compounds were significantly elevated in the endometriosis group (n = 15, p less than 0.05). This suggests an increase in the peritoneal fluid levels of thromboxane A2 and prostacyclin, both of which could act on tubal smooth muscle and interfere with tubal function. Such altered tubal function might explain the phenomenon of endometriosis-induced infertility when there is no direct damage to the reproductive organs.