Screwed acetabular cups, applied in total hip replacements, generate stresses in the surrounding bone during implantation (prestresses). The effect of these prestresses on the endurance of the hip replacement are unknown. The prestresses in the acetabulum were examined both experimentally, using strain gauge techniques, and numerically, using the finite element method. It was found that the prestresses were of the same order of magnitude, if not larger, than the stresses due to the hip reaction force during one-legged stance. In some cases, the prestresses even approximated the ultimate tensile strength of cortical bone. The prestresses seemed to have a strong dependence on the outer shape of the cup, rather than on the flexibility of the cup or whether the cup had a self-cutting thread or not. Furthermore, it was found that the prestresses are not very susceptible to stress relaxation due to the visco-elastic behaviour of bone. This means that prestresses will remain present over long periods of time. So even when a patient has resumed normal daily activities, the prestresses will still play an important role in the overall stress distributions around the acetabulum. Due to the interaction of prestresses and stresses due to normal loading, the primary stability of a metal-backed screwed cup is better guaranteed than the primary stability of an all-polyethylene screwed cup.