A clinical case that suggested the involvement of blood coagulation cascades in the inflammation reaction induced by leakage of ovarian endometriotic cysts was encountered. Here, a rapid elevation in plasma D-dimer level (119.6 micro g/ml) was observed within 5 h after onset of ovarian cyst rupture. By contrast, the plasma fibrinogen level fell below normal range (105 mg/dl). During the following few days, the plasma D-dimer level fell acutely to approximately normal, while the fibrinogen level gradually increased to a high value (590 mg/dl). In addition, the anti-thrombin III level was slightly reduced on the day after rupture, though the concentration of blood platelets remained unchanged. Three days later, laparoscopic surgery showed the surface of the posterior wall of the uterus and peritoneum of the pelvic wall to be extensively covered with the leaked content of a right ovarian endometriotic cyst. Pathological examination confirmed the diagnosis of endometriotic cyst. In the endometrial stromal region, deposit of D-dimer was detected by immunohistochemical staining. These findings suggest that fibrin-derived degradation products in endometriotic cysts can affect coagulation cascades when they leak into the peritoneal cavity; they may also induce local inflammatory reactions, causing pelvic pain and/or extension of the endometriotic lesion.