To evaluate the effect of IVF-ET on the hemostatic system.Prospective clinical study.Apparently healthy age-matched women of the hospital staff at various stage of the menstrual cycle.Twenty-five women involved in a IVF-ET program at the Department of Obstetrics and Gynecology, Montpellier University Hospital.Twenty-six hemostasis parameters evaluated repeatedly in patients undergoing IVF-ET.Blood cell-dependent hemostasis parameters and plasmatic coagulation factors, determined at pituitary desensitization, maximal E 2 level, and P plateau.Activation of the hemostatic system is evidenced at the P plateau, when D-dimers and fragments 1 + 2 of the prothrombin levels rose dramatically. At E 2 peak, no significant modification of hemostasis markers was noted.The present results indicate that ovarian hyperstimulation may induce hemostasis activation at the P plateau. The role of supraphysiologic sex hormone levels on the hemostatic system requires further investigation.