In taking the history in these atypical instances of myositis ossificans one should pay particular attention to previous trauma. If there is history of trauma, careful timing of the progression of the pathological process is helpful in diagnosis. Finally, and most important, in these atypical cases as well as the typical cases, zone phenomena are present. There is a central, undifferentiated zone impossible to distinguish from sarcoma. This zone merges into oriented osteoid formation and finally, into well formed bone in the periphery of the lesion. If these zones are present, they are of paramount importance in making a diagnosis of a benign process and may save the patient from mutilating and needless surgery.