The intraosseous pressure in the femur and tibia near the knee and in the internal saphenous vein at knee level was measured in fifty-three patients with suspected knee lesions. There were four groups: with and without degenerative osteoarthritis and with and without aching rest pain of the knee region. Low intraosseous pressures were found in patients with neither osteoarthritis nor rest pain, and in half the patients with osteoarthritis but without rest pain. Low pressures were found in the tibia, but very high intraosseous pressures were found in the femur in most patients with osteoarthritis and rest pain. Patients with no osteoarthritis but with rest pain mostly had high pressures in both the tibia and the femur.

1. In fifteen patients with unilateral osteoarthritis of the hip bilateral measurements of the intraosseous pressure of the femoral neck and determination of femoral vein pressure were done simultaneously. These pressure examinations were followed by bilateral intraosseous phlebography of the proximal part of the femur.

2. In a second series of fifteen patients the intraosseous pressures of the femoral head and neck were measured simultaneously before operation for osteoarthritis.

3. The pressure in the femoral vein was equal on the two sides. The intraosseous pressure in the femoral neck was always higher in the arthritic hip than on the unaffected side. In hips with osteoarthritis the pressure in the femoral head was higher than the pressure in the neck.

4. Intraosseous phlebography indicated a state of intramedullary venous engorgement in osteoarthritis. The normal channels for venous drainage from the femoral head and neck were not visible in the phlebographs from the arthritic side. Instead, drainage took place through descending intramedullary vessels to the trochanteric region and down into the femoral shaft. The emptying of intraosseous contrast material from the arthritic hip was delayed.

5. The phlebographs indicated that the abnormally high intraosseous pressure observed in osteoarthritis is caused by a high resistance to flow across the cortex of the proximal part of the femur.

6. The aching rest pain typical of severe osteoarthritis was noted only in patients with intraosseous femoral neck pressure above 40 millimetres of mercury, an indication that this type of pain is caused by intramedullary hypertension. The decrease of arteriovenous pressure difference, caused by increase of resistance to venous outflow, is probably accompanied by disturbances of nutritive flow. This "venous ischaemia" may play an important role for the structural changes of cancellous bone in osteoarthritis.