Objective: to compare GnRH antagonist and agonist protocols regarding the outcome and rate of OHSS. Subjects: women who were referred to the infertility center of Beheshti hospital, Isfahan, Iran, for ART . 136 patients were randomly allocated to 2 groups: 67 patients were treated with GnRH agonist, 69 patients were treated with GnRH antagonist. Inclusion criteria: age ≤35, FSH level ≤10 IU/liter. Exclusion criteria: hyperprolactinemia, thyroid dysfunction, uterine abnormality, severe endometriosis. Group A, a daily dose of Buserelin 500 µg given SC and commenced 21st day of cycle, and vaginal ultrasonography continued onward until estradiol on the 2st day of cycle. Group B, GnRH antagonist was administered. Ovarian stimulation was started on the 2nd day of the cycle by SC injection of 75 IU of FSH daily. On the 6th day of stimulation, 0.25 mg Cetrorelix was initiated. Based on the ovarian response detected by ultrasonography, gonadotropin dose was adjusted in both groups. Administration of Buserelin and Cetrorelix was continued until the time of HCG injection. When at least 3 follicles with a mean diameter of 18 mm were developed, HCG 10,000 IU was injected IM. At this stage, endometrial thickness was studied Trans vaginal ultrasonographically, and after 36 hours, oocyte retrieval was performed. After IVF or ICSI, Cyclogest 800 mg was prescribed daily to provide luteal phase support and continued till the activity of FHR was confirmed TVS. Sixteen days after the oocyte retrieval, serum HCG level was checked to determine chemical pregnancy. TVS was carried out for clinical determination of pregnancy.