Decreased periovulatory human chorionic gonadotrophin (HCG) concentrations have been shown to be associated with diminished fertilization rates. This study evaluated if intra-follicular HCG concentration may be related to body mass in 247 IVF patients using their own oocytes and 58 patients receiving donor oocytes, and evaluated if such a relationship might affect IVF outcome. A significant inverse correlation (r = −;0.353, P < 0.001) was observed between the body mass index (BMI) and intra-follicular HCG concentration. The mean HCG concentrations were significantly decreased (P < 0.001) in patients with a BMI >30 kg/m 2 compared with patients with a BMI of 20–30 kg/m 2 or BMI <20 kg/m 2 (17.6 versus 45.1 and 52.5%, respectively). The clinical pregnancy rates (P < 0.001) and embryo quality (P < 0.05) were significantly different for the three groups. In donor oocyte recipients, the pregnancy rate was significantly decreased (P < 0.0001) for recipients with a BMI >25 kg/m 2 compared with those with a BMI from 21–25 kg/m 2 and BMI <21 kg/m 2 (43.8 versus 72 and 76.5%, respectively). These data indicate that intra-follicular HCG concentration is inversely related to BMI, and may be related to a concurrent decrease in embryo quality and pregnancy rates.