Abstract: A hip joint prosthesis comprises a femoral unit and an acetabulum unit. The acetabulum unit has a rigid cap of a material having a low coefficient of friction, defining a cavity which receives the ball head of the femoral unit, and an outer shell having a rigid radially inner part accommodating the cap and a resilient radially outer part anchored in the pelvic bone and radially deformable. For imparting resiliency to the outer part, it may be fractionated into a plurality of finger-like angular sectors each joined to the inner part by an annular zone of reduced thickness. The femoral unit may also be rendered resilient, for instance by locating a plurality of corrugated rings in recesses distributed along the length of its shaft.