During free tissue transfer, much effort is made to keep patients normothermic. It is feared that hypothermia, which is common in patients having such operations, can induce vasospasm leading to stasis and ultimately thrombogenesis. The present study was undertaken to determine the effect of core body temperature on the survival of free flaps in an animal model. Rats were anesthetized with inhaled isoflurane and randomly assigned to one of the following four core temperature groups: 34 degrees C, 35 degrees C, 37 degrees C, and 39 degrees C (n = 10 animals per group). Bilateral groin free flaps were then performed (n = 20 flaps per group) while each animal was maintained at the temperature of its assigned group. Flap survival was evaluated on postoperative day 5 by a blinded observer. The best flap survival occurred in the 34 degrees C group, with an overall flap survival rate of 95%. There was a statistical difference between the survival rate of the combined 34 degrees C and 35 degrees C group (survival rate 90%, n = 40) and the combined 37 degrees C and 39 degrees C group (survival rate 67.5%, n = 40; P = 0.027). Hypothermia may have a beneficial effect on the success of free tissue transfer.