The pathophysiology of skin flaps raised on nonexpanded and expanded skin was studied in pigs. Skin expansion was achieved by gradual inflation of a subcutaneously implanted silicone tissue expander with sterilized isotonic saline over a period of 4 to 5 weeks. Comparisons were made of the skin flap viability and capillary blood flow among random acute and delayed skin flaps and skin flaps constructed on skin pockets implanted with noninflated or inflated tissue expanders. The extents of skin flap viability and capillary blood flow were similar among delayed skin flaps and skin flaps constructed on skin pockets but were significantly higher than those of acute random skin flaps raised on normal skin. It was concluded that skin viability and capillary blood flow in skin flaps raised on expanded skin were not compromised and the regulation of blood flow in skin flaps raised on nonexpanded or expanded skin pockets was similar to that seen in delayed skin flaps. The mechanisms of the delay phenomenon was discussed.