During the ten-year period ending in 1978, 10,500 conventional total hip arthroplasties were performed at the Mayo Clinic; dislocation developed after 331 (3.2 per cent) of these procedures. Cross correlations of the data were performed using multivariate analysis. This analysis showed that previous surgery on the hip was the most significant of the factors predisposing to dislocation, the incidence doubling from 2.4 per cent (in hips without previous surgery) to 4.8 per cent (in hips with previous surgery) (p less than 0.001). The dislocation rate was 2.3 per cent after an anterolateral approach and 5.8 per cent after a posterior approach (p less than 0.01). The size of the head of the femoral component was not a strongly influential factor. The incidence of dislocation was 17.6 per cent in the hips that had osteotomy and avulsion of the greater trochanter, compared with 2.8 per cent in those in which the trochanteric osteotomy united (p less than 0.001). Reoperation for instability of the hip was performed in a third of the patients, but in 31 per cent of the patients whose hip was reoperated on the instability persisted after the revision.