A standard photoplethysmograph, modified to differentiate between arterial and venous vasculature in skin tissue, has been used to measure arterial supply in 100 unipedicle skin flaps in 25 pigs. A datum transform from a light intensity scale to a relative arterial blood density scale has been introduced and empirically labelled the vascular coefficient (VC). Statistical analysis of the VC data shows a significant difference between necrotic and surviving flap group mean data as early as 24 hours. Additional analysis in which a normal ogive was fitted to the sample VC data distributions suggests that statistically significant predictions of the probability of eventual flap necrosis can be made using VC measurements obtained immediately postoperatively. It is concluded that the modified photoplethysmograph in conjunction with the VC data transform, constitutes a usable measuring technique for skin flap viability prediction in the experimental animal.