Bisphenol A (BPA) is a synthetic chemical used in the manufacture of materials present in many common consumer products. In experimental animals, BPA caused oocyte aneuploidy and reduced production of oestradiol. In a prospective cohort study, we investigated the association between urinary BPA concentrations and ovarian response among women undergoing in vitro fertilization (IVF) at the Massachusetts General Hospital (MGH) Fertility Center. The geometric mean of two specific-gravity (SG) adjusted urinary BPA concentrations collected during each IVF cycle was used as the cycle-specific BPA exposure level. BPA concentrations were measured using online solid phase extraction coupled to isotope dilution-high-performance liquid chromatography-tandem mass spectrometry. Peak serum oestradiol was measured using the Elecsys Estradiol II immunoassay kit. Multivariable mixed effect models and Poisson regression models adjusting for correlation between multiple IVF cycles in the same woman were used to evaluate the association between urinary BPA concentrations and ovarian response, adjusting for age, BMI and day 3 follicle stimulating hormone (FSH) levels, a clinical measure of ovarian reserve. Urinary BPA concentrations were measured in 84 women (mean age 35.6 years) undergoing 112 IVF cycles; 23 women (27%) contributed more than one IVF cycle. BPA concentrations ranged from <0.4 to 25.5 microg/L (geometric mean 2.52 +/- SD 3.2); 15% of urine samples had concentrations <0.4 microg/L. Peak serum oestradiol levels correlated with the total number of oocytes retrieved per cycle (r = 0.65, p < 0.001). For each log unit increase in SG-BPA, there was an average decrease of 12% (95% CI: 4, 23%; p = 0.007) in the number of oocytes retrieved and an average decrease of 213 pg/ml (95% CI: -407, -20; p = 0.03) in peak oestradiol. BPA was detected in the urine of the majority of women undergoing IVF, and was inversely associated with number of oocytes retrieved and peak oestradiol levels.