Thirty-five total hip arthroplasties done in twenty-five patients with protrusio acetabuli secondary to rheumatoid arthritis were reviewed. There was an average follow-up of 4.3 years, with a range of three to seven years. The results were rated as excellent or good in 66 per cent, fair in 26 per cent, and poor in 8 per cent. Although 100 per cent demonstrated cementbone interface demarcation around the acetabular component, only 10 per cent showed progression of the line of demarcation to two millimeters and one had acetabular loosening with migration. Eight per cent showed femoral loosening or subsidence; 8 per cent, calcar resorption; and 6 per cent, a receding cortex with cystic changes. Twenty-three per cent had nonunion of the greater torchanter after trochanteric osteotomy. Type-III cement-bone interface demarcation was present around the acetabular component was positioned one centimeter superiorly or medially beyond the anatomical position, as estimated by the method described. In thirteen hips in which the acetabular component was positioned within five millimeters of the anatomical position, no Type-III demarcation was present. Better fixation and position of the acetabular component is achieved by the use of a bone graft or a special titanium perforated-sheet mesh, or both, or by an acetabular shell. The use of three wires improved trochanteric fixation.