We conducted a study to evaluate differences between patients who had total hip arthroplasty (THA) for posttraumatic osteoarthritis (OA) and patients who had THA for primary OA. Using a prospective database, we followed 3844 patients who had THA for OA. Those who had THA for secondary causes of hip OA, developmental hip dysplasia, or inflammatory processes were excluded. Of the remaining 1199 patients, 62 (63 fractures) had THA for posttraumatic OA after previous acetabular or proximal femur fracture fixation, and 1137 had THA for primary OA and served as the control group. In the posttraumatic OA group, mean time between fracture repair and conversion to THA was 74 months. Compared with the control patients, the posttrauma patients lost more blood, were transfused more units of blood, had longer operating room times, and had more complications (all Ps < .001). Posttrauma patients had a mean follow-up of 4.44 years and a mean postoperative modified Harris Hip Score of 81.3 at latest follow-up. Of these patients, 12.5% required revision a mean of 3.5 years after initial arthroplasty. THA in patients with posttraumatic hip OA after an acetabular or proximal femur fracture is a longer and more complicated procedure with a higher rate of early failure.