To elucidate characteristic changes of plasma hemostatic markers in ovarian hyperstimulation syndrome (OHSS) cycles. Prospective study. The IVF-ET program of the Department of Obstetrics and Gynecology, The University of Akita, School of Medicine. Forty cycles of 40 IVF patients, including 12 cycles in which a severe form of OHSS occurred. Blood samples were taken during IVF treatment to determine the levels of blood markers that reflect activation of the coagulation and fibrinolytic systems. Thrombin-antithrombin III complexes, plasmin-alpha 2 antiplasmin complexes, and other hemostatic markers related to the coagulation and fibrinolytic system activation. In the OHSS cycles, the levels of thrombin-antithrombin III and plasmin-alpha 2 antiplasmin complexes in the plasma began to rise within a few days after hCG administration and demonstrated significantly higher levels during the midluteal phase. In OHSS cycles with pregnancy, elevation of these markers continued for > or = 3 weeks after the onset of disease. There were some characteristic changes in OHSS cycles in other hemostatic markers, such as a decrease in the levels of antithrombin III and prekallikrein and shortened activated partial thromboplastin time. These data demonstrate the status of the coagulation and fibrinolytic systems in OHSS cycles and provide insight into the mechanism of activation in the hemostatic system.