The frequency of radiological demarcation of the cement-bone junction in the acetabulum after total hip replacement has been examined in 141 Charnley low-friction arthroplasties followed for an average of 10.1 years. Sixty-nine per cent showed demarcation of various degrees and 9.2 per cent of the series showed evidence of progressive migration of the socket. The vast majority of cases with demarcation were symptomless. In most cases where demarcation was accompanied by migration the operation notes suggested a technical explanation and in three cases low-grade sepsis was responsible. The fact that nearly 30 per cent of cases showed no demarcation even after 10 years supports the idea that there is no fundamental defect in the principle of employing cement in the acetabulum. Better surgical technique may increase the number of cases showing no demarcation.