In twelve patients undergoing total hip replacement, the femoral intramedullary pressure was measured. It was transitorily high when the femoral prosthesis was press-fit, higher when the methylmethacrylate was forced into the cavity, and highest when the femoral prosthesis was finally positioned. Venting proximally was ineffective but a distal vent prevented the rise in pressure. This may be good prophylaxis against embolization or cardiac arrest, or both.