1. A clinical study has been made of heterotopic ossification in 273 patients with paraplegia of traumatic and non-traumatic origin treated at the Liverpool Paraplegic Centre over a period of twelve and a half years.

2. The literature is reviewed and theories of etiology are discussed.

3. Etiological factors have been studied. Prominent among these is inadequacy of early treatment leading to urinary infection and to the formation of pressure sores.

4. It is concluded that there is no effective treatment for established heterotopic ossification.

5. The importance of prophylactic treatment is stressed. Special emphasis is placed on adequate primary treatment, correction of hypoproteinaemia and early mobilisation.