Firmly attached screw-in polyethylene acetabula which had been implanted for between 16 and 54 months were explanted after autopsies and subjected to macroscopic, radiologic and histologic examination. Metaplasias were seen around the threads and on the floor of the acetabulum. Their tendency to ossify represents an attempt at secondary stabilization. Due to the low stability of the polyethylene this causes increased wear on the floor of the acetabulum. The small defects in the polyethylene found in the threads, resembling damage done by mice, may be a sign of biodegradation. In view of the tissue reactions pointed out, the material stability of the polyethylene needs to be improved or implantation must be restricted to a very limited range of indications.