Objective: To evaluate the efficacy of superovulation and IUI versus no treatment for infertility associated with minimal or mild endometriosis. Design: Randomized trial. Setting(s): London Health Sciences Centre, University Campus, The University of Western Ontario, London, Ontario; and Royal Victoria Hospital, McGill University, Montreal, Quebec, Canada. Patient(s): Three hundred eleven cycles in 103 couples in whom minimal or mild endometriosis was the sole identified subfertility factor. Intervention(s): Superovulation with FSH and IUI. Main Outcome Measure(s): Live birth. Result(s): Live birth followed 14 of 127 (11%) superovulation and IUI cycles and 4 of 184 (2%) no-treatment cycles. The odds ratio was 5.6 (95% confidence interval 1.8 to 17.4) in favor of superovulation and IUI. Conclusion(s): Treatment with superovulation and IUI was associated with superior outcome both by crude live-birth rates and proportional hazard analysis. (C) 1997 by American Society for Reproductive Medicine.