A total of 35 cases of periarticular new bone formation (PNBF) was observed among 160 patients with coma following severe craniocerebral trauma. All cases were associated with blunt trauma and none with penetrating wounds. Only 6 of 500 cases of acute non-traumatic hemiplegia developed PNBR, and all 6 of them followed craniotomy, brain surgery and coma. New bone formation first appeared mainly between 50 and 120 days after craniocerebral injury with prolonged coma. Three-quarters of the patients with PNBF showed involvement of the shoulder joint, but this was not associated with previous subluxation. Metabolic studies were done in some patients; no disturbances were found in the metabolism of calcium, phosphorus or alkaline phosphatase. The pathologic process of PNBF seemed to stabilize some 6 to 8 months following trauma, and surgery after this period produced functional improvement in the 3 patients in whom it was tried. No satisfactory pathophysiological explanation has been found for the phenomenon of PNBR. Prolonged coma is common to all patients who suffered from PNBF and is probably an etiologic factor. The absence of PNBF in cases of cerebrovascular accident with subluxations of the gleno-humeral joint and intensive physiotherapy seems to contradict the suggestion of microtrauma as an etiological factor.