This prospective, double-blind, randomized controlled trial was designed to evaluate the efficacy of a culture temperature of 36.5°C versus 37°C on human embryo development in vitro . A total of 412 women undergoing IVF were randomized to two groups: the oocytes and embryos of the interventiongroupwereculturedat36.5°C;thoseofthecontrolgroupwereculturedat37°C.Althoughnosignificanteffectofculturetemperaturewasobserved onpregnancyorimplantationrates,differenceswerefoundinembryodevelopment.Embryocultureat36.5°Cwasassociatedwithasignificantlyhigher cleavage rate (OR 1.6, 95% CI 1.03 to 2.51), but a lower fertilization rate, fewer high-quality embryos on day 3, a lower blastocyst formation rate on day 5, and fewer high-quality and cryopreserved blastocysts (OR 0.87, 95% CI 0.78 to 0.98), (OR 0.60, 95% CI 0.53 to 0.69), (OR 0.85, 95% CI 0.75 to 0.97), (OR 0.5, 95% CI 0.44 to 0.56) and (OR 0.77, 95% CI 0.68 to 0.88), respectively, compared with 37°C. On the basis of these results, and in the absence of data on the optimal temperature for each stage of embryo development in vitro , we recommend continuation of the use of 37°C for human embryo culture.