Between 1971 and 1979, 680 low-friction arthroplasties of the hip were performed in 598 patients. The average duration of follow-up was twelve years and eight months. Sixty-one acetabular cups had loosening as seen on roentgenograms eighteen years postoperatively, resulting in a total cumulative probability of loosening of 19 per cent, according to survivorship analysis. In twenty-nine cups, the loosening appeared within ten years after the operation (early loosening) and in thirty-two, more than ten years after the operation (late loosening). Early loosening was associated with deficient structure of the bone of the acetabulum, a previous congenital dislocation of the hip, acetabular fracture, or acetabular protrusion in all instances (p < 0.01). Late loosening was associated with the depth of acetabular wear. Of the thirty-two cups that had more than two millimeters of wear, eighteen (56 per cent) had loosening on the roentgenograms (p < 0.001). In hips that had early loosening, migration was the most frequent finding, and its rate of progression was higher than in hips that had late loosening (p < 0.001). In late loosening, a complete bone-cement radiolucency of more than two millimeters was the most frequent finding. Clinical failure was seen in twenty-two (76 per cent) of the twenty-nine cups that loosened early and in nine (28 per cent) of the thirty-two cups that loosened late. The probability of extensive resorption of bone necessitates close observation of patients who have early loosening, while a reasonable period of observation is possible for those who have late loosening.