In 18 patients who underwent total hip replacement, the intramedullary pressure in the femur was measured during the insertion of the femoral prosthesis component. Half the patients had a pressure release hole (diameter 4.5 mm) drilled into the medullary canal at the distal end of the femur, the other half not. In the patients without a venting hole, the intramedullary pressure increased transiently to a median of 390mmHg during the insertion of the femoral stem, while those with a venting hole only showed an increase to 23mmHg. A drop in arterial oxygen tension of 2.2kPa and in thrombocytes of 51 x 10(9)/1 was found in those without a venting hole, while the corresponding values in those with a venting hole were 0.9kPa and 20 x 10(9)/1. Furthermore, a significant correlation was demonstrated between the increase in intramedullary pressure and the drop in oxygen tension and in blood platelets. No significant change in blood pressure was measured during operation in either group.