Several studies have examined the association between body mass index and infertility. We compared body mass index in 597 women diagnosed with ovulatory infertility at seven infertility clinics in the United States and Canada with 1,695 primiparous controls who recently gave birth. The obese women (body mass index > or = 27) had a relative risk of ovulatory infertility of 3.1 [95% confidence interval (CI) = 2.2-4.4], compared with women of lower body weight (body mass index 20-24.9). We found a small effect in women with a body mass index of 25-26.9 or less than 17 [relative risk (RR) = 1.2, 95% CI = 0.8-1.9; and RR = 1.6, 95% CI = 0.7-3.9, respectively). We conclude that the risk of ovulatory infertility is highest in obese women but is also slightly increased in moderately overweight and underweight women.