Twenty-four hip replacements were performed with use of a medial protrusio technique to stabilize the fit of a hemispherical metal shell in the acetabulum in nineteen patients who had dysplasia of the hip. All of the hips were followed for a minimum of five years (average, seven years; range, five to thirteen years). Six of the hips were type I, seven were type II, eight were type III, and three were type IV according to the criteria of Crowe et al. The acetabular cup was implanted with the medial aspect of its dome beyond the Kohler line (drawn from the ischium along the ilioischial line) in all hips. An autogenous graft sculpted from the femoral head was used to cover 15 to 30 percent of the superolateral portion of the cup in one type-I hip, four type-III hips, and one type-IV hip. The need for these six bone grafts could have been avoided by reaming two to three millimeters more medially or by allowing 20 percent of the superolateral portion of the cup to be uncovered. Sixty to 84 percent of each bone graft was resorbed, effectively leaving the superolateral portion of the cup uncovered.The amount of the surface of the cup that was beyond the Kohler line averaged 41 percent for the six type-I hips, 43 percent for the seven type-II hips, 41 percent for six of the type-II hips, and 44 percent for one of the type-IV hips. Crossing of the ilioischial and iliopubic lines was noted on the radiographs of two type-III and two type-IV hips. Radiographs of two type-I hips and one type-II hip showed 7 to 17 percent of the surface of the dome of the cup through the internal pelvic wall (beyond the iliopubic line).None of the twenty-four metal shells were revised. A reoperation was performed on two hips to exchange a worn polyethylene insert, and three femoral components that had been fixed without cement were revised because of mechanical loosening. Wear averaged 0.26 millimeter per year in the fourteen hips that had a titanium femoral head and 0.09 millimeter per year in the nine hips that had a cobalt-chromium femoral head. The remaining hip had a ceramic femoral head, and the wear rate was 0.09 millimeter per year.The medial protrusio technique is a predictable, reproducible method for obtaining fixation of a porous-coated, hemispherical acetabular component in a dysplastic acetabulum. The technique permits the use of a porous-coated (bone-ingrowth) component; avoids the use of support bone graft and thereby reduces the operative time; facilitates rehabilitation by permitting earlier weight-bearing of the hip; and permits the use of a modular bearing surface, which may allow future exchange of only this surface rather than revision of the entire acetabular component because of excessive wear.