Heterotopic ossification(HO) is a formation of ectopic bone around joints in the soft tissues. It often occurs after the central nervous system injuries, burns, and joint replacements. It causes an important morbidity in the rehabilitation population. The etiology, pathogenesis, prophylaxis, and treatment of HO are still unclear even though there have been many investigation on the prevention of HO among the patients with spinal cord injury. The prevention effect of EHDP on HO formation still remains controversial. A randomized clinical trial was performed to assess the prevention effect of EHDP on HO formation in 73 patients (control group: 40 cases, study group: 33 cases) as a prospective study. EHDP was given to the study group for 12 weeks; 20 mg /kg /day for 2 weeks followed by 10 mg/kg/day for 10 weeks. The patients were followed up with the serial physical examinations, serum alkaline phosphatase levels, and plane radiographs for one year period after the injury. The results revealed that HO developed in eight cases(20.0%) among the control group and two cases(9.1%) among the study group. There was no significant difference in the incidence of HO between two groups(p>0.05). The mean duration from the injury to the detection of HO was 113 days. Twelve of 15 HO sites(80.0%) were detected within the 6 months after injury. Seven patients developed the HO in one site, two in two sites, and one in three sites. The sites of HO were 6 in hips, 4 in knees, 2 in shoulders, 2 in pelvis, and one in elbow. The results of this study do not support the prevention effect of EHDP on HO formation among the patients with spinal cord injury.