This study examined the infleunce of three types of boot-sock systems on incidence and severity of foot blisters. Participants were 357 men undergoing U.S. Marine recruit training at Parris Island, South Carolina. Each participant was assigned to one of three treatment groups. The first group wore the standard military boot sock consisting of a wool-cotton-nylon-Spandex combination. The second group wore the standard military boot sock with a thin inner or liner sock consisting of polyester. The third group wore a very thick, dense, prototype outer sock consisting of a wool-polypropylene combination over the same liner sock as the second group. Foot blister incidence was lower among participants who wore the the prototype boot sock and liner than among those wearing the standard military sock (40 vs. 69%, p < 0.001) or among subjects wearing the standard military sock with liner (40 vs. 77%, p < 0.001). Foot blisters and cellulitis severe enough to require medical attention occurred with greater frequency in the standard military sock group compared to both the prototype boot-sock group (24 vs. 11%, p = 0.02) and the standard military sock with liner group (24 vs. 9%, p < 0.01); there was no difference between the latter two groups. Blister reduction was most apparent in the early weeks of recruit training. The standard military sock with a polyester liner reduced the incidence of severe blisters, but the dense sock with the polyester liner reduced the overall incidence of blisters as well as the incidence of severe blisters.