A comparison of pressure reducing properties of alternating air, static air, and water mattress overlays was conducted with 57 patients in a surgical intensive care unit. Sacral and heel pressures in both recumbent and semi-Fowler's positions were tested for each surface using a repeated measures design. Mean pressures for the alternating air mattress were significantly higher than pressures with other surfaces, regardless of position or site. There were significant main effects for position and site, with higher pressures in the semi-Fowler's position and at the sacral site. A significant Interaction between surface, site, and position was found. Pressure sores developed in eight patients, but the incidence was not significantly different across groups. A pressure measuring device constructed from available clinical materials proved to be both sensitive and reliable. The findings suggest alternating air overlays should be avoided, and that positioning and periodic position change to reduce sacral pressures for patients requiring prolonged upper body elevation is important.