The authors retrospectively reviewed 378 total hip arthroplasties(THR) in placed in 321 patients between January 1987 and November 1991. We examined all of the radiographs at a minimum of 6 months after operation and classified the severity of the heterotopic wsification by Brooker et al. The purpose of this study was to investigate the incidence of heterotopic ossification(HO) after THR and to find out the risk factor for HO development in Korea. The overall incidence of heterotopic ossification was 43Fo(162 cases). The degrees were class I in 108 hips(67%), class E in 37 hips(22%), class K in 14 hips(3.7%), and class F in 4 hips(3%). The incidence were 68 hips(37.8%) in avascular necrosis, 23 hips(53.8%) in osteoarthritis, 39 hips(56,5%) in fracture, 12 hips(48Fo) in tuber- culous arthritis, 7 hips(58.3%) in pyogenic' arthritis, 5 hips(83.4%) in ankylosing spondylitis, 4 hips(50%) in rheumatoid arthritis and 3 hips(75%) in fused hip. Three cases of class 1Y belonged to tuberculous arthritis group. The incidence of heterotopic ossification had not a significant correlation with age, sex, preop hip function and weight. The Harris hip scores of class N were significantly lower than any other classes. In conclusion, the incidence of the heterotopic ossification was 162(43%) in 378 cases. There was a predisposition to develop severe heterotopic ossification among patients with tuberculous arthritis.