OBJECTIVE To determine the relationship between ovarian follicular response (number and diameter of follicles) to ovulation induction agents and pregnancy. METHODS Retrospective cohort study.Hospital-based center of reproductive medicine. METHODS Three hundred twenty-two clinically diagnosed infertile couples undergoing 1483 cycles of ovulation induction and IUI. Oral and injectable fertility drugs were administered for 2-10 cycles; ultrasound follicular measurements were recorded around time of ovulation; hCG was administered when follicles were 18-20 mm or more in diameter; IUI was performed 36-40 hours after administration of hCG. METHODS A first positive pregnancy test as indicative for conception. Women with follicular diameters >or=20 mm were less likely to become pregnant as compared to women with diameters between 15.00 and 19.99 mm (risk ratio [RR] = 0.58, 95% confidence interval [CI] = 0.35, 0.97). Women >or=30 years of age were half as likely to become pregnant as compared to women <30 years (RR = 0.51, 95% CI = 0.30, 0.85). Likelihood of pregnancy increased by 21% for each prior pregnancy (RR = 1.21, 95% CI = 1.00, 1.47). CONCLUSIONS Maternal age, gravidity, and follicular diameters around the time of insemination are prognostic factors in the likelihood of pregnancy.