We reviewed the results of reoperation in ninety-five patients who had acute subluxation (ten patients) or dislocation (eighty-five patients) of the hip after conventional cemented total hip-replacement arthroplasty. Postoperatively, fifty-eight patients (61 per cent) had no subsequent dislocation or subluxation. Seven of thirty-seven patients who had had recurrent dislocation had occasional subluxation during follow-up. Of the remaining thirty patients in whom instability persisted after the reoperation, twenty-eight had at least one dislocation, and nine had bothersome subluxation. Ten of these thirty-seven patients had another operation for the persistent instability. The causes of instability were classified as malrotation of the component, disruption of the trochanteric-abduction mechanism, impingement, or multiple and unknown, and appropriate treatment was provided. The component was revised in forty-five patients, revision and advancement of the trochanteric component was done in twenty-five patients, and impinging bone or cement was removed from six patients; a combination of these procedures was done in nineteen patients. Over-all, fifty-eight procedures (61 per cent) were successful (no additional subluxations or dislocations). We concluded that the results of operative treatment for an unstable total hip replacement can be optimized when a precise determination of the cause of the instability is made and appropriate measures are applied.