The use of fluorescein as a predictor of skin viability in the rat flap model of avascular skin necrosis is well known in the field of plastic surgery. Inherent in this model are several faults that must be corrected if it is to become a valid standard. Fluorescence readings are one of the main variables in flap models. By carefully shaving rat backs with a double-edged razor, taking fluorescence readings in a uniformly darkened space, carefully tattooing the exact extent of both the bright and the mottled areas of fluorescence on the flap, and placing a plastic barrier between the flap and the body to prevent any part of the flap from acting as a graft, we were able to quantitatively predict which portions of a flap would survive and which would die after fourteen days. The bright fluorescent area (mean = 11.99 cm2, SEM ± 0.53 cm2) is equal to the amount of tissue that will survive (mean = 11.91 cm2, SEM ± 0.50 cm2; NS), while the sum of the nonfluorescent area and the mottled fluorescent area (mean = 27.692, SEM ± 0.52 cm2) is equal to the amount of tissue that will die (mean = 27.99 cm2, SEM ± 0.52 cm2; NS).