The effect of subcutaneously injected recombinant human basic fibroblast growth factor (bFGF) was studied in an arterial skin flap model on the ear of the hairless mouse. Fifty-three male, hairless mice were randomly assigned to 4 groups and pretreated in two different time intervals with different doses of human bFGF. Microvascular perfusion of the skin flaps was determined over a 5-day period by means of intravital microscopy after intravenous injection of the fluorescence marker fluorescein isothiocyanate—dextran (M, 150,000). Human bFGF (2,700 ng) injected 6 days before flap creation could not improve perfusion of the flap (n = 10) when compared with controls. However, when applied 18 days before flap creation (n = 13), the same dose resulted in a significant reduction of nonperfused tissue at day 5 after flap creation (12.3% vs 26.8%, p < 0.01). Eighteen-day pretreatment with 1,200 ng (n = 10) and 480 ng (n = 10) had no significant effect on skin flap perfusion. We conclude, therefore, that successful pretreatment with bFGF for prevention of skin flap necrosis is time and dose dependent.