The radiographic data of 48 hips in 40 patients with painful, progressive protrusio acetabuli treated with cemented total hip arthroplasty and medial acetabular reinforcement using autologous or homologous bone grafts were reviewed. Twelve separate measurements were performed on the preoperative, postoperative, and most recent radiographs of each case to determine which measurements were the most useful in diagnosing protrusio acetabuli and in following preoperative and postoperative migration. The minimum follow-up period was 10 years (mean, 12.3 years). Vertical migration and horizontal distance, two measurements utilizing an X-Y coordinate system based on the acetabular teardrop, were found to be most useful. The cases with postoperative progression demonstrated vertical migration more often than medial migration.