To determine the effect of urinary versus recombinant FSH on platelet function and hemostatic variables in women undergoing controlled ovarian hyperstimulatio n cycles. Randomized clinical study. Major universitybased infertility and in vi tro fertilization unit and hemostasis laboratory. Ten healthy women (in vitro st udy), and 24 women undergoing routine controlled ovarian hyperstimulation cycles (in vivo study), randomly assigned to receive either urinary (u-FSH) or recomb inant gonadotropin (r-FSH). In vitro study: effect of preincubation of plasma w ith u-FSH or r-FSH, in the presence or absence of estradiol, on platelet funct ion and coagulation parameters. In vivo study: Changes in platelet function and coagulation parameters after treatment with u-FSH or r-FSH during controlled o varian hyperstimulation cycles. Platelet aggregation and ATP release, activated protein C resistance ratio, free protein S. In vitro study: Platelet aggregation and ATP release were significantly inhib-itedbyu-FSH relative to r-FSH in bo th the presence and absence of estradiol (P=.047). In vivo study: Platelet funct ion was significantly inhibited after treatment with u-FSH (P=.05) but not with r-FSH. In both studies, small changes of minor clinical significance were note d in activated protein C resistance and free protein S levels. The different pla telet response to u-FSH and r-FSH may have clinical implications in selected p atients, especially those at risk of thromboembolic complications, in decisions regarding the appropriate medication for controlled ovarian hyperstimulation cyc les. To determine the effect of urinary versus recombinant FSH on platelet function and hemostatic variables in women undergoing controlled ovarian hyperstimulatio n cycles. Randomized clinical study. Major university based infertility and in vi tro fertilization unit and hemostasis laboratory. Ten healthy women (in vitro st udy) , and 24 women undergoing routine controlled ovarian hyperstimulation cycles (in vivo study), randomly assigned to receive either urinary (u-FSH) or recomb inant gonadotropin (r-FSH). In vitro study: effect of preincubation of plasma w ith u- FSH or r-FSH, in the presence or absence of estradiol, on platelet funct ion and coagulation parameters. In vivo study: Changes in platelet function and coagulation parameters after treatment with u-FSH or r-FSH during controlled o varian hyperstimulation cycles. Platelet aggregation and ATP release, activated protein C resistance ratio, free protein S. In vitro study: Platelet aggregation and ATP release were significantly inhib -bybyu-FSH relative to r-FSH in bo th the presence and absence of estradiol (P = .047). In vivo study: Platelet funct ion was significantly inhibited after treatment with u-FSH (P = .05) but not with In both studies, small changes of minor clinical significance were note d in activated protein C resistance and free protein S levels. The different pla telet response to u-FSH and r-FSH may have clinical implications in selected patients, especially those at risk of thromboembolic complications, in decisions regarding the appropriate service for controlled ovarian hyperstimulation cyc les.