We investigated the effect of exogenous basic fibroblast growth factor (bFGF) on the viability of random skin flaps in rats. Thirty-six rats were divided into three groups of 12 animals: a saline control group and two bFGF-treated groups. Caudally based random skin flaps were raised on the backs of the rats. In the control group, saline was instilled under the flap after skin closure. Twelve other rats were treated with bFGF (20 micrograms) just after skin closure. The remaining 12 rats were treated twice with bFGF (20 micrograms), both just after the operation and 48 hours later. There was a significant improvement in viability in the bFGF-treated groups on postoperative day 7 compared with the control group. Total adenine nucleotide levels in the flaps on postoperative day 10 were significantly higher in the group receiving two doses of bFGF than in the other groups. Thus, bFGF can improve the viability of endangered skin flaps.