Although the physiological effects of pressure on tissue have been demonstrated in the animal model, little is known about its effect in ill, elderly patients who are at risk for pressure ulcers. This study describes the pattern of dermal blood flow during a period of constant, low-level, compressive pressure in this population of patients. Dermal blood flow was measured over the trochanter of 16 elderly (>60 years) subjects who were defined as at risk for pressure ulcer development by the Braden Scale for Predicting Pressure Ulcer Risk. Using a laser-Doppler velocitometer, blood flow at baseline and during 60 minutes of leftside lying on an air mattress were measured. Mean blood flow at baseline was 0.79 (SD 0.43). Following 60 minutes of compressive pressure, mean blood flow was 0.65 (SD 0.87). Blood flow trac-ingsduring the 60-minute period of continuous, compressive pressure revealed an inconsistent pattern of response; the flow increased, decreased, or showed no change. This distribution of responses suggests that significantly more variability in blood flow response exist in at-risk individuals than was previously believed.