Pressure ulcers are frequently seen dermal lesions, but little is known about their pathophysiology. It is generally assumed that prolonged tissue pressure impairs blood circulation thus causing ischemic damage to tissue. Therefore, subcutaneous oxygen partial pressure was measured to confirm this hypothesis. In the past, various authors have conducted tests on healthy subjects to determine oxygen partial pressure transcutaneously during periods not exceeding 20 minutes. All found a decrease at susceptible sites, e.g., the sacrum. The present study was the first one to measure oxygen partial pressure subcutaneously above the sacrums of four test subjects during a period of 5 hours. In all cases, the values first decreased to a minimum of 37% of baseline before they returned to the initial values. This observation is in contradiction to former studies, which start from the assumption of critical ischemia due to interface pressure, measured on healthy volunteers too.