This experimental study was designed to identify the etiology of pressure ulcers in a surgical sample and to evaluate a special OR mattress overlay in preventing pressure ulcer development. Surgical patients (N = 413) were randomized to receive "usual perioperative care" or the new mattress overlay. Over six postoperative days, 89 patients (21.5%) developed pressure ulcers, primarily stage I. Only 2% developed stage II or IV ulcers. Patients with ulcers were statistically older, had diabetes, were smaller in body mass, had lower Braden Scale scores on admission, and used the new mattress overlay (P < .02). Pressure ulcers that presented as "burns" or ecchymosis did not deteriorate to stage III or IV ulcers during the study. The mattress overlay was not effective in preventing pressure ulcer development.