In this prospective study, a technique of positioning the acetabulum by bony landmarks of the pelvis in the standing position was developed using a standing lateral preoperative roentgenogram with the X-ray tube centered over the trochanter. Since 1984, 441 total hip arthroplasties (THAs) were done through the posterior approach with a 1.14% dislocation rate through 1988 and no dislocations in 1989. To prevent impingement and dislocation, it was determined that the safest range for cup position was 30 degrees-50 degrees abduction and 20 degrees-40 degrees flexion from the horizontal. To measure postoperative cup position, a standing true lateral roentgenogram of the operated hip allowed direct measurement of cup flexion and was reproducible within 10 degrees. No special instruments are necessary for this technique, which can be used with any THA system.