The incidence, time of onset, and role of exercise in the progression of heterotopic ossification were documented retrospectively in burn patients. In 12 of 1,066 patients (1.2%) consecutively admitted to a burn center, the abnormal bone formed posteriorly around the elbow joint. The initial signs were localized joint pain and rapid decrease in range of motion, and the average time of onset was 12 weeks after thermal injury. Prior to the diagnosis of heterotopic ossification, all patients were managed with an exercise program of active and active-assisted movements. In patients who persisted with passive and active-assisted range of motion, especially beyond the range of pain-free movements, the ossification progressed to complete ankylosis and required surgical intervention to remove the heterotopic bone. On the other hand, postoperative patients and patients who followed a program of active exercise within the pain-free range gained excellent range of motion. It was concluded that passive stretching of the periarticular structures during the acute phase of heterotopic bone formation is detrimental to the final outcome.