The purpose of this study was to investigate whether endothelial cell procoagulant activity (PCA), which triggers the extrinsic coagulation pathway, is involved in thrombosis in patients with ovarian hyperstimulation syndrome (OHSS). PCA in OHSS sera was measured by an activation of tissue factor expression on cultured human umbilical vein endothelial cells (HUVEC). The substance responsible for the expression of PCA in sera was partially purified and characterized. The results obtained were as follows. (1) OHSS sera contained significantly (p < 0.05) higher PCA than in controlled women (during the follicular phase). Fetal cord sera contained very strong PCA. (2) There was no correlation between PCA and the level of either estradiol (E2) or progesterone (P4) in OHSS sera. (3) The substance that expresses PCA in fetal cord sera was heat-labile with high affinity for heparin sulfate and had a molecular weight of approximately 100,000. These results support these conclusion: (1) PCA is involved in thrombosis in patients with OHSS and (2) neither the serum level of E2 nor that of P4 is associated with PCA.