A custom-designed femoral prosthesis was implanted with cement and a standard acetabular component was used to treat nineteen severely dysplastic hips in fourteen consecutively managed patients. Components that had been custom-designed with the use of plain radiography were used because the anatomical reconstructive goals could not be achieved with commercially available implants. These goals were to match the offset of the femoral head and the length of the lower limb with those on the normal side for patients who had unilateral involvement and to provide an average (thirty to forty-millimeter) offset with equal limb lengths for patients who had bilateral involvement. A retrospective clinical and radiographic analysis was performed. The diagnoses included coxa vara (one hip), congenital dislocation (twelve hips), achondroplasia (three hips), and spondyloepiphyseal dysplasia (three hips). The mean age at the time of the reconstruction was forty-nine years (range, twenty-two to seventy-three years), and the mean duration of follow-up was fifty-seven months (range, twenty-seven to 108 months). In five hips, bone-grafting of the acetabulum was needed to obtain superolateral coverage. The clinical result was excellent in eighteen hips and good in one. No revisions have been performed to date. Two femoral components were possibly loose radiographically. One was associated with a definite loosening of the acetabular cup. In addition, one other cup was possibly loose. There was a 100 per cent rate of survival if only a revision procedure was considered as a failure.(ABSTRACT TRUNCATED AT 250 WORDS)