Heterotopic ossification, a roentgenographically diagnosed sequel to surgical procedures on the hip, originally was considered to be a 'cause of pain and of failure after arthroplasty. Some authors speculated on the role of surgical trauma, bone dust, and bleeding. But, No cause for the ossification has yet been determined. This study was designed to evaluate the incidences and the factors which might predispose to the development of ossification and to correlate these factor after 116 hip arthroplasty in 91 patients who were treated at our hospital from January 1985 to December 1990. The results are as follows; l. Overall incidence of heterotopic ossification is 27%(31 hips in 116 hips). The degrees of heterotopic ossification(Brooker et al. classivication) were grade I 12 hips (10%), grade II 8 hips(7%), grade III 8 hips(7%) and grade IV in 3 hips(3%). 2. There were no significant differences of the incidence of heterotopic ossification between sexes, age groups, and type of arthroplasty. 3. Risk factors of heterotopic ossification are as follows. 1) Preoperative diagnosis ankylosing spondylits. 2) Preoperative high alkaline phosphatase level. 3) Previous operation on ipsilateral hip. 4. Postoperative treatment with anti-inflammatory drugs and diphosphonate effectively prevents the formation of heterotopic ossification.