Ovarian hyperstimulation syndrom(OHSS) is a serious complication of controlled overian hyperstimulation(COH).identifying risk factors of OHSS seems to be an important task because of two main reasons:increasing the incidence of infertility and the fact that around 1-7% of COH leade to OHSS.this study is a case-control  study conducted in infertility center at Dr.shariati hospital.75 patients,condidate for assisted reproductive technology(ART),who developed OHSS after COH were selected as the case group and 120 matched patients without OHSS were selected as control group.factors such as body mass index(BMI),serum estradiol level on the day of HCG administration,presence of polycystic ovarise and patients,age were investigated,the incidence of OHSS was higher in younger patients.the incidence of PCO in the case group(OHSS)(37.3%) was higer than that of control group(20.8%).incidence of BMI>30 was higher in the case group as compared with the control group,the serum estradiol level on the day of HCG administtation in the OHSS patients was higher than that of control group (1534+_1177 pg/ml and 1124+_739pg/ml respectively).regarding the increasing of infertility incidence and the development of new methods of COH,detecting the risk factors of OHSS plays an important role in reduction of this dangerous and even fatal disease.