The aim of this study was to assess the impact of isolated obesity on the outcome of intracytoplasmic sperm injection (ICSI). A total of 775 patients undergoing 1113 ICSI cycles were categorized on the basis of body mass index (BMI): group I (BMI 18.5-24.9 kg/m(2); normal weights n = 627 cycles), group 2 (BMI 25.0-29.9 kg/m m(2); overweight; n = 339 cycles) and group 3 (BMI >= 30 kg/m(2); obese; n = 147 cycles). Sixty-three (10.0%) cycles in group 1, 53 (15.6%) cycles in group 2 and 26 cycles (17.7%) in group 3 were cancelled (P < 0.05 for group 1 versus groups 2 and 3). Despite the significantly higher total gonadotrophin consumption in groups 2 and 3 compared with group 1, the mean serum oestradiol level on the day of human chorionic gonadotrophin administration was significantly higher in group 1 (P < 0.05). The number of cumulus-oocyte complexes, metaphase II oocytes, and two-pronucleated oocytes were significantly lower in group 3 compared with group 1 (P < 0.05). However, fertilization rate, the mean number of embryos transferred, the mean number of grade 1 embryos transferred, clinical pregnancy, implantation, multiple pregnancy and miscarriage rates were comparable among the three groups. The rate of cycles with cryopreservable embryos was significantly lower in groups 2 and 3 compared with group 1 (P < 0.05).