Heterotopic ossification (HO) occurs in 4-49% of patients with spinal cord injury, but the cause of the complication has not been established. The aim of this study was to investigate clinical factors related to the occurrence of HO. The incidence of HO was determined in 91 consecutive patients with traumatic lesions of the spinal cord who had been admitted to the National Spinal Injuries Centre for management and rehabilitation. Clinical data were analysed. Clinically apparent HO occurred only in 10 of 56 patients in whom the start of passive movements to their paralysed limbs was delayed until 7 days or more from time of injury. The findings of this study are consistent with the view that HO occurs as a result of trauma induced by passive movements carried out on joints where contractures have started to develop.