OBJECTIVE To describe the clinical experience of our center with in vitro fertilization (IVF) in unstimulated cycles and to provide a comparison to stimulated cycles. METHODS Spontaneous ovulatory cycles were triggered with human chorionic gonadotropin in the midcycle, and 78 aspirations for IVF were performed, with the remainder of the IVF cycle proceeding in a standard manner. METHODS The IVF program of the University of Southern California and the California Medical Center, Los Angeles, California. METHODS Spontaneously ovulatory women (n = 46) with predominantly pelvic factor as their principal cause of infertility, under the age of 40, and no male factor. METHODS Human chorionic gonadotropin administration in midcycle, follicle aspiration, IVF, and embryo transfer. METHODS Embryo implantation and pregnancy. RESULTS Seventy-eight follicle aspirations resulted in 11 clinical (14%) and 9 ongoing (12%) pregnancies. The per embryo implantation rate was 13% clinical and 11% ongoing. There was no decrease in per cycle pregnancy rates (PRs) for up to three unstimulated cycles. CONCLUSIONS Unstimulated IVF is a viable alternative to stimulated cycles with PRs approximately one half those of stimulated cycles. It is reasonable to offer patients up to three cycles of unstimulated IVF without expecting a decrease in PRs.