The aim of this study was to evaluate the clinical efficacy of body mass index (BMI) as a predictor of in vitro fertilization and embryo transfer (IVF-ET) outcomes. Two hundred twenty-three IVF-ET cycles in 164 patients under 37 yr using GnRH agonist long protocols were included in this retrospective study. All of the selected cases were divided into two groups by a cutoff of 24 kg/m2 and these two groups were compared in regard to the outcomes of IVF-ET. There were no significant differences between group 1 (BMI <24 kg/m2) and group 2 (BMI > or = 24 kg/m2) in age, basal serum FSH level, estradiol (E2) level and endometrial thickness on hCG day, number of retrieved oocytes and transferred embryos. However, higher doses of gonadotropins were used in group 2 (30.8+/-12.7 ampoules vs. 35.4+/-15.3 ampoules, p=0.051). The clinical pregnancy rate was significantly lower in group 2 (25.9% vs. 10.5%, p=0.041) and implantation rate tended to be lower in group 2 (12.7% vs. 6.8%, p=0.085). BMI > or = 24 kg/m2 can be a candidate prognosticator of IVF-ET outcomes.