Objective-To determine whether a spontaneous luteinizing hormone (LH) surge influences the pregnancy and miscarriage rate in women with unexplained infertility undergoing controlled ovarian hyperstimulation (COH) without in vitro fertilization. Design-Retrospective cohort study. Setting-Ovulation Induction Clinic, Royal Hospital for Women, Sydney, Australia. Patients-145 patients with unexplained infertility who underwent 374 cycles of COH. Methods-Two types of ovarian stimulation protocols were used: human menopausal gonadotrophin (hMG) alone or hMG and clomiphene citrate (CC). A spontaneous LH surge occurred in 54% of the total cycles. All patients received human chorionic gonadotrophin, whether or not a spontaneous LH surge occurred. All cycles were covered by natural intercourse. Main Outcome Measures-Cycle pregnancy rate and miscarriage rate in cycles with or without a spontaneous LH surge. Results-The cycle pregnancy rate of the LH surge group was significantly higher than that of the no LH surge group for CC/hMG cycles (16.4% and 4.3% respectively, p = 0.02) but not for hMG alone cycles (12.8% and 10% respectively, P > .05). The miscarriage rate was not significantly different between the LH surge group and no LH surge group 11 either the CC/hMG cycles (30% and 75% respectively, P > .05) or the hMG alone cycles (22% and 38% respectively, P > .05). Conclusions-In women with unexplained infertility undergoing COH with CC/hMG, the occurrence of a spontaneous LII surge is a favorable event associated with a significantly increased pregnancy rate. The data showed a lower miscarriage rate, but there was insufficient power to confirm or refute this result.