This study evaluates the effect of ultrasound-assisted preconditioning on critically ischemic flaps. Ninety-eight Sprague Dawley rats were randomly divided into eight groups. Control, surgical delay, ultrasound and sham groups were designed. Modified McFarlane flaps were raised on the back of rats, and flap survival rate was assessed on post-operative day 14 in control, ultrasound and sham groups. Bipedicled flaps were created in the surgical delay group, and 14 d after delaying, all skin flaps were elevated. Statistically, flap survival rates of all ultrasound groups were significantly higher than the control group. Plasma vascular endothelial growth factor levels were increased in all ultrasound groups. Vessel counts did not show any difference between the groups. This study shows that the preconditioning by ultrasound can be used to improve the viability of ischemic skin flaps almost at a level close to the surgical delay. (C) 2019 World Federation for Ultrasound in Medicine & Biology. All rights reserved.