BACKGROUND End-stage osteoarthritis secondary to total congenital dislocation or severe congenital dysplasia of the hip (class B or C according to the system of Eftekhar or type 2 or 3 according to the system of Hartofilakidis et al.) in adults presents special problems with regard to reconstruction of the hip. The purpose of the present study was to assess the intermediate-term results associated with the use of a porous ingrowth acetabular component for the treatment of these difficult cases. METHODS We performed a prospective study of a consecutive series of twenty-one patients (twenty-four hips) who had had a primary total hip arthroplasty with use of a hemispherical acetabular component that was inserted without cement and fixed with screws. No patient was lost to follow-up. Three patients (four hips) died, of causes unrelated to the total hip arthroplasty, before a minimum duration of follow-up of five years. None of these patients had had revision. Of the remaining eighteen patients (twenty hips), fifteen were women and three were men. Ten hips had total dislocation, and ten had severe dysplasia. RESULTS After an average duration of follow-up of eighty-three months (range, sixty-four to 102 months), the average Harris hip score was 90 points (range, 68 to 97 points). No patient had revision, loosening, or migration of the acetabular component; pelvic osteolysis; or a continuous radiolucent line at the mesh-bone interface of the acetabular component. The average rate of polyethylene wear was 0.08 millimeter per year (range, zero to 0.21 millimeter per year). CONCLUSIONS The porous ingrowth acetabular component that was used in the present study functioned well at the time of the intermediate-term follow-up of this group of patients who had marked congenital dysplasia or total dislocation of the hip. The use of this component decreased the need for structural acetabular grafts. This component appears to perform as well as larger components of this design that have been assessed after similar durations of follow-up.