Nine patients who had spinal cord injury and were receiving diphosphonate therapy for established neurogenic heterotopic ossification in 14 hips were followed up for an average of 14 months. Diphosphonate therapy at a dosage of 10 mg/kg/day when prescribed an average of 26 days after diagnosis did not prevent radiographically evident progression of heterotopic ossification. The interval of one to 30 days between clinical diagnosis and institution of diphosphonate treatment did not appear to affect the duration of radiographic progression or the final radiographic grade at maturity. Disodium etidronate therapy did not prevent the appearance of heterotopic ossification in three previously unaffected hips, although the final radiographic grade was mild. The radiographic progression of heterotopic ossification averaged 5.3 months. No clinically detectable side effects were attributable to the medication at the dosage prescribed after an average of 14 months of therapy. Due to the radiographic progression noted in patients who received 10 mg/kg/day, diphosphonate has been increased to dosages of 20 mg/kg for six months after early diagnosis.