To help to determine the natural history of residual dysplasia of the hip after skeletal maturity, we followed the status of the contralateral hip in 286 patients who had had a total hip replacement for osteoarthrosis secondary to dysplasia. The initial radiographic findings in seventy-four patients in whom advanced osteoarthrosis later developed in the contralateral hip were compared with those in forty-three patients who had reached the age of sixty-five years without having had severe osteoarthrosis. No patient in whom the hip functioned well until the age of sixty-five years had had a center-edge angle of less than 16 degrees, an acetabular index of depth to width of less than 38 per cent, an acetabular index of the weight-bearing zone of more than 15 degrees, uncovering of the femoral head of more than 31 per cent, or an acetabulum in which the most proximal point of the dome had been at the lateral edge (zero peak-to-edge distance).