Polyethylene wear and the subsequent development of periprosthetic osteolysis are the major factors limiting the longevity of total hip arthroplasties. A minority eventually loosen, but no clinically applicable system exists for accurate early prediction of failure. The relationship between acetabular wear and the development of loosening, osteolysis, and revision was investigated in 235 Charnley low friction arthroplasties. The average age of the patient at surgery was 31.7 years (range, 17-39 years), and the duration of followup averaged 234 months (19.5 years; range, 74-364 months). Total wear averaged 2.1 mm (range, 0-7.2 mm), and the average wear rate was 0.11 mm per year (range, 0-0.55 mm/year), with the wear rate of revised components being twice that of surviving ones (0.19 mm/year versus 0.09 mm/year). The prevalence of osteolysis (33 hips, 14%) and of acetabular and femoral component loosening and revision rose significantly with increasing wear. Osteolysis also was associated significantly with femoral component loosening and revision, but the presence of calcar changes was not (90 hips, 38%). Twenty-five year survivorship exceeded 90% for arthroplasties with a wear rate less than 0.1 mm per year, but 20-year survivorship of acetabular components with a rate greater than 0.2 mm per year was below 30%, and none survived 25 years. For every additional millimeter of wear, the risk of acetabular revision in any one year increased by 45% and for the femur increased by 32%.