Purpose :Women with ultrasonic evidence of polycystic ovaries are at higher risk of ovarian hyperstimulation syndrome (OHSS). We focused on investigating a possible association of the (TTTTA) n microsatellite polymorphism in the promoter of the CYP11A gene with OHSS during controlled ovarian hyperstimulation (COH). Methods :We evaluated 58 patients at high risk of OHSS (study group) and 58 control patients undergoing controlled ovarian hyperstimulation.