Laser Doppler flowmetry was used to examine the relationships between skin blood flow and pressure ulcer development. Blood flow in the skin over the iliac and sacral bony prominences was measured intraoperatively in 24 consecutive patients undergoing lengthy surgical procedures. Patients who did not develop pressure ulcers postoperatively had a 500% mean increase in blood flow during the procedure as compared with the preoperative levels. Blood flow levels decreased during surgery in patients who developed pressure ulcers postoperatively. There was no statistical difference in the lengths of surgery between the two groups. These results suggest that the body's failure to increase blood flow in response to extended pressure during surgery may contribute to pressure ulcer development.