Mechanical risk factors in loosening of the socket were radiographically analyzed in 123 Charnley arthroplasties performed during the period 1969-1982 for coxarthrosis secondary to congenital dislocation. The socket was placed as near the true acetabulum as possible without extensive soft-tissue release and without reinforcement of the acetabular roof by bone grafting. The rate of socket loosening was 19 percent using progression of a radiolucent line at the bone-cement interface to a width of greater than 1 mm as the criterion for loosening. Different predictors of loosening were analyzed by stepwise logistic regression analysis. Lack of lateral bony support for the socket was the prime predictor. The next most important predictors were the preoperative degree of hip dislocation and the position of the socket in relation to the true acetabulum.