To determine if the immunotherapy with corticosteroids would improve pregnancy rate in infertile patients with ovulatory factor and patients with unexplained infertility who undergo superovulation with IUI. Prospective, controlled study. Division of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynecology, University of Ulsan, Seoul, Korea. Nine-one infertile patients with ovulatory factors and 78 patients with unexplained infertility who underwent superovulation with IUI. The prevalence of autoantibodies (antinuclear antibody, lupus anticoagulant, anticardiolipin antibody, antidouble-stranded DNA antibody) was elevated significantly in patients with unexplained infertility compared with patients with ovulatory infertility (20.5% versus 3.3%). Forty-five patients with ovulatory factor underwent 72 cycles of superovulation with IUI, receiving corticosteroids. Thirty-eight patients with unexplained infertility underwent 75 cycles of superovulation with IUI, receiving corticosteroids. Forty-six patients with ovulatory factor who underwent 66 cycles of superovulation with IUI and 40 patients with unexplained infertility who underwent 75 cycles of superovulation with IUI served as controls, not receiving corticosteroids. In patients with ovulatory factor, there were no significant differences between the corticosteroid treatment group and control group in clinical pregnancy rate (38.9% versus 33.3%) or in spontaneous abortion rate (14.3% versus 13.6%). In patients with unexplained infertility, there was a significantly higher clinical pregnancy rate per cycle in the corticosteroid treatment group, with 45.3% (34/75) compared with 29.3% (22/75) in the control group but no difference between the corticosteroid treatment and control groups in spontaneous abortion rate (17.6% versus 13.6%). There were no side effects due to the use of corticosteroids.