A decreased intraabdominal fibrinolytic activity has been proposed as an etiological factor in the development of endometriosis. To test this hypothesis plasminogen, fibrinogen, α2-antiplasmin, α2-macroglobulin, plasminogen activator (t-PA) and its inhibitor and the degradation products of fibrin were determined in the peritoneal fluid of 25 patients with and 45 patients without endometriosis. No significant difference was found for any of the parameters. Therefore, a role for the fibrinolytic system as an etiological factor in the development of endometriosis is unlikely. A high concentration of t-PA was found in comparison to normal blood levels, leading to a high concentration of fibrin degradation products in the peritoneal fluid, indicating an active system of intraabdominal fibrinolysis. A high concentration of fibrin degradation products further indicates the presence of fibrinogen and its turnover secondary to thrombin action.