Studying the effect of GnRH antagonist administration on the day of hCG to cases of IVF/ICSI with estradiol level above 5000 ng/dl for protection of ovarian hyperstimulation syndrome. Prospective study. Sixty patients undergoing controlled hyperstimulation COH, for IVF/ICSI using long agonist and E2 level on the day of hCG, are above 5000 ng/dl, 52 patients received single dose of cetrorelix 0.25 mg on the day of hCG, and 8 patients received two doses of 0.25 mg/day cetrorelix started one day before the day of hCG. There was no significant difference regarding patients BMI, number of stimulation days, recombinant FSH dose, and number of retrieved oocytes. Clinical pregnancy rate was 76.6% (46/60), in patients received single dose of antagonist PR were significantly higher 80.7% (42/52) versus 50% (4/8) in patients received two doses p = 0.047. Live birth rate was 50% (30/60), abortion rate was 20% (12/60), and preterm delivery was 20% (12/60). Mean E2 was 6853.2 ng/dl. Six patients developed moderate ovarian hyperstimulation OHSS (6/60) 10% and no cases of severe OHSS. GnRH antagonist administration on the day of hCG in cases undergoing IVF/ICSI with long agonist protocol is effective in protection of OHSS and does not affect the clinical pregnancy rate nor live birth rate.