Modified natural-cycle IVF has a lower pregnancy rate per started cycle as compared with IVF with ovarian stimulation due to, for example, premature ovulation. Indometacin administered before ovulation prevents follicle rupture. Therefore, addition of indometacin may improve the effectiveness of modified natural-cycle IVF. This double-blind, randomized, placebo-controlled trial with indometacin or placebo in 120 women aged 27-36 years compared the number of patients without premature ovulation as compared with the number of patients with one or more ovulations in a maximum of six cycles. Indometacin had no significant influence on the probability of a premature ovulation in patients during the six cycles (OR 2.38, 95% CI 0.94-6.04). A subgroup analysis showed a significant influence of indometacin in decreasing the probability of a premature ovulation in cycles without LH surge at the day of human chorionic gonadotrophin administration (OR 8.29, 95% CI 1.63-42.3, P = 0.009). Although this study could not detect a significantly lower ovulation rate in the indometacin group versus the placebo group, the data suggest that a subgroup of patients without LH surge prior to oocyte retrieval might benefit from indometacin in modified natural-cycle IVF. (C) 2013, Reproductive Healthcare Ltd. Published by Elsevier Ltd. All rights reserved.