['To determine whether conversion of gonadotropin/IUI cycles at high risk of high-order multiple pregnancies to IVF yields a pregnancy rate high enough to warrant the added intervention.', 'Case-control study.', "Brigham and Women's Hospital assisted reproductive technology program.", 'Seventy-seven patients converted from gonadotropin/IUI to IVF, 77 consecutive age and attempt number-matched controls (sequential controls [SCs]), and 77 consecutive age-, attempt-, and E(2)-matched controls (E(2) controls [ECs]).', 'Gonadotropin/IUI cycles with exuberant responses were converted to IVF (cases) to avoid cycle cancellation and high-order multiple pregnancies.', 'Pregnancy rates, delivery rates, E(2) levels, follicle and oocyte number, and fertilization and implantation rates.', 'Compared with SCs and ECs, cases had more follicles (16.3 +/- 0.6 vs. 13.3 +/- 0.9 and 14.4 +/- 0.9) and higher E(2) at hCG administration (1,951 +/- 93 vs. 1,568 +/- 96 and 1,939 +/- 89 pg/mL). Delivery rates among the three groups (45.5% vs. 32.5% and 39.0%) did not differ significantly. Despite the transfer of fewer embryos in cases than in controls (2.5 +/- 0.1 vs. 3.1 +/- 0.1 and 2.9 +/- 0.1), three triplet pregnancies occurred in cases and three in controls.'] Does conversion of high-response gonadotropin intrauterine insemination cycles to in vitro fertilization result in excellent ongoing pregnancy rates?