Objective-To compare basal (cycle day 3) follicle-stimulating hormone (FSH) level, clomiphene citrate challenge test (CCCT), gonadotropin-releasing hormone agonist stimulation test (GAST), and mean ovarian volume estimation by ultrasound for predicting the subsequent ovarian response. Design-Prospective, randomized, clinical study. Setting-Referral university hospital. Patients-One hundred and forty-four women with unexplained infertility undergoing their first ovulation induction treatment with low-dose recombinant FSH. Interventions-Patients were randomized into four groups. Basal FSH levels were evaluated in group I (n = 36). Clomiphene citrate challenge test (CCCT) and gonadotropin-releasing hormone agonist stimulation test (GAST) were carried out in group II (n = 36) and group III (n = 36), respectively. Transvaginal ultrasound was performed for ovarian volume measurements in group IV (n = 36). In the subsequent cycle, all women received ovulation induction therapy with recombinant FSH. Main Outcome Measures-Number of mature (greater than or equal to 14 mm) follicles and the number of recombinant FSH ampules required for successful ovulation induction. Results-Ovarian volume estimation by transvaginal ultrasound, compared to the other three tests, had the most powerful positive correlation with the number of mature follicles (r = 0.84, P < .0001) and the most powerful negative correlation (r = -0.75, P < .0001) with the amount of recombinant FSH used per cycle. Conclusion-Mean ovarian volume estimation by transvaginal ultrasound might be more useful than basal FSH values, CCCT, and GAST for predicting ovarian response to low-dose recombinant FSH treatment.