To look for evidence of a fibrinolytic insufficiency as a cause of adhesion formation. Retrospective and prospective study. University medical center. Retrospective study: 50 patients undergoing laparoscopy, divided into patients with and without endometriosis. Prospective study: 18 patients undergoing infertility surgery involving a second- look laparoscopy. During all surgical procedures, adhesions were scored, and peritoneal fluid and plasma were collected. Parameters of the fibrinolytic system were measured to establish a possible relation with the presence and formation of adhesions. In patients with endometriosis and adhesions, significantly higher peritoneal fluid concentrations were found for plasminogen activator inhibitor1 (PAI- 1), tissue plasminogen activator (tPA), and plasminogen, compared with patients with endometriosis but without adhesions. In the prospective study, initial peritoneal PAI- 1 concentrations correlated significantly with the extent of adhesion formation (rs = 0.49) and adhesion- improvement scores (rs = - 0.52). Also, the change in concentration of tPA and fibrinogen from the initial surgical procedure to the second- look laparoscopy correlated significantly with adhesion- improve- ment scores (tPA: rs= 0.50; fibrinogen: rs = - 0.64). This first prospective study in humans adds further weight to the hypothesis that adhesions are caused by an insufficiency in peritoneal fibrinolytic activity. Plasminogen activator inhibitor1 is a potential marker for the identification of patients at risk for developing adhesions. To look for evidence of a fibrinolytic insufficiency as a cause of adhesion formation. Retrospective and prospective study. University medical center. Retrospective study: 50 patients undergoing laparoscopy, divided into patients with and without endometriosis. Prospective study: 18 patients undergoing infertility surgery involving a During all surgical procedures, adhesions were scored, and peritoneal fluid were also scored, and peritoneal fluid were collected from mice. higher peritoneal fluid concentrations were found for plasminogen activator inhibitor 1 (PAI-1), tissue plasminogen activator (tPA), and plasminogen, compared with patients with endometriosis but without adhesions. In the prospective study, initial peritoneal PAI- 1 concentrations correlated significantly with the extent of adhesion formation (rs = -0.49) and adhesion-improvement scores (rs = - 0.52). Also, the change in concentration of tPA and fibrinogen from the initial surgical procedure to the second- look laparoscopy correlated significantly with adhesion- improve scores (tPA: rs = - 0.50; fibrinogen: rs = - 0.64). This first prospective study in humans further further weight to the hypothesis that adhesions are caused by an insufficiency in peritoneal fibrinolytic activity. Plasminogen activator inhibitor 1 is a potential marker for the identification of patients at risk for developing adhesions.