A longitudinal double-blind study was conducted to determine the effect of sock fiber composition on the frequency and size of blistering events in long-distance runners. Thirty-five long-distance runners participated in this study. Two different socks were tested, which were identical in every aspect of construction except fiber composition. One test sock was composed of 100% acrylic fibers, and the other test sock was composed of 100% natural cotton fibers. The results showed that acrylic fiber socks were associated with fewer blistering events and smaller blisters (mm2), when compared directly to cotton fiber socks.