We compared intraosseous pressure, partial pressure of oxygen and carbon dioxide, and histopathology in 10 hips with arthrosis and in seven hips with nontraumatic necrosis of the femoral head. Hypoxia of the subchondral bone was present in both conditions. In arthrosis the intraosseous pressure was normal, whereas intraosseous hypertension was present in necrosis. Partial pressure of carbon dioxide was normal in both conditions. Identical signs of subchondral medullary and trabecular necrosis were found in both conditions.