A high density polyethylene (RCH 1000) femoral head prosthesis was inserted in 5 large dogs. Periodic clinical and roentgenographic evaluations, and histopathological examination of the replaced joints as well as biomechanical evaluation of the prosthetic joints were done at intervals from 3 weeks to 15 months. HDP is NOT suitable for movable weight-bearing prosthesis articulating with cartilage or bone. Wear of the cartilage and the polyethylene occurred within a few weeks of function and became extensive proportionally to the length of use. The accumulating wear particles of HDP caused a significant foreign body granulomatous reaction which resulted in erosion of bone adjacent to the acetabulum in one hip.