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We describe three distinct types of congenital hip disease in adults. The first type is dysplasia, in which the femoral head is contained within the original true acetabulum. The second type is low dislocation, in which the femoral head articulates with a false acetabulum, the inferior lip of which contacts or overlaps the superior lip of the true acetabulum, giving the appearance of two overlapping acetabula. The third type is high dislocation, in which the femoral head has migrated superoposteriorly and there is no contact between the true and the false acetabulum. We describe and classify the acetabular abnormalities and deficiencies found with these three types.If the anterior, posterior, and superior aspects of the acetabular component cannot be covered during a total hip arthroplasty because of a deficient acetabulum in an adult who has congenital hip disease, we advocate an acetabuloplasty technique (which we have named a cotyloplasty) that involves medial advancement of the acetabular floor by the creation of a controlled comminuted fracture of its medial wall, autogenous bone-grafting, and the implantation of a small acetabular component with cement.This procedure was performed in sixty-six patients (eighty-six hips). Forty-nine of the hips had a high dislocation, thirty-one had a low dislocation, and six were dysplastic. Two to fifteen years (mean, seven years) after the operation, the clinical and radiographic results were satisfactory. Only two acetabular components needed to be revised for aseptic loosening, at 5.3 and 7.5 years postoperatively. Moreover, the cumulative success rate for the acetabular components was 100 per cent at five years and 93.2 per cent at ten years.