BACKGROUND Gonadotropin-releasing hormone (GnRH) antagonist protocols for pituitary down regulation in in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI) allow the use of GnRH agonists for triggering final oocyte maturation. Currently, human chorionic gonadotropin (HCG) is still the standard medication for this purpose. The effectiveness of triggering with a GnRH agonist compared to HCG measured as pregnancy and ovarian hyperstimulation(OHSS) rates are unknown. OBJECTIVE To compare the effectiveness of a GnRH agonist with HCG for triggering final oocyte maturation in IVF and ICSI patients undergoing controlled ovarian hyperstimulation in a GnRH antagonist protocol followed by embryo transfer. METHODS We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE , EMBASE, the National Research Register, the Medical Research Council's Clinical Trials Register, and the NHS Centre for Reviews and Dissemination database. We also examined the reference lists of all known primary studies and review articles, citation lists of relevant publications and abstracts of major scientific meetings. METHODS All randomised controlled studies (RCTs) reporting data comparing clinical outcomes for women undergoing IVF and ICSI cycles and using a GnRH agonist in comparison with HCG for final oocyte maturation triggering. METHODS Two review authors independently assessed trial quality and extracted data. RESULTS We identified 11 RCTs (n = 1055). Eight studies assessed fresh autologous cycles and three studies assessed donor-recipient cycles. In fresh-autologous cycles, GnRH agonist was less effective than HCG in terms of the live birth rate per randomised woman (OR 0.44, 95% CI 0.29 to 0.68; 4 RCTs) and ongoing pregnancy rate per randomised woman (OR 0.45, 95% CI 0.31 to 0.65; 8 RCTs). For a group with a 30% live birth or ongoing pregnancy rate using HCG, the rate would be between 12% and 22% using an GnRH agonist. Moderate to severe ovarian hyperstimulation syndrome (OHSS) incidence per randomised woman was significantly lower in the GnRH agonist group compared to the HCG group (OR 0.10, 95% CI 0.01 to 0.82; 5 RCTs). For a group with a 3% OHSS rate using HCG the rate would be between 0% and 2.6% using GnRH agonist. In donor recipient cycles, there was no evidence of a statistical difference in the live birth rate per randomised woman (OR 0.92, 95% CI 0.53 to 1.61; 1 RCT). CONCLUSIONS We do not recommend that GnRH agonists be routinely used as a final oocyte maturation trigger in fresh autologous cycles because of lowered live birth rates and ongoing pregnancy rates. An exception could be made for women with high risk of OHSS, after appropriate counselling.