Methods: a prospective randomized study was designed to evaluate clinical and endocrine outcome in two different coasting protocols. Women (n=120) under controlled ovarian hyperstimulation with GnRH agonist protocol at the risk of OHSS (≧20 follicles >12 mm development with E2> 4000 pg/ml) randomized into two groups. Group I (n=60), withdrawing GnRH agonists and continued low dose r-FSH 75 IU. Group II (n=60), GnRH antagonist administration and continued low dose r-FSH 75 IU. When E2＜ 3000 pg/ml, hCG was given. Oocyte retrieval was performed 36 h after hCG administration. The primary outcome measures were the decreased levels of estradiol (E2) and the days of coasting. The secondary outcome measures were number of oocytes retrieved, pregnancy rate and the incidence of OHSS.