A review of the cases of thirty-five patients with os odontoideum, the largest series reported to date, supports the concept that trauma is the etiology of the abnormality. Most of the patients had roentgenographic instability of the affected spinal segment, and were treated with spine fusion. Eleven patients had lesions attributable to injuries received before they were four years old, although surgical treatment usually was delayed for one to eight years. Nine patients had documented roentgenographic evidence of a normal odontoid process prior to the development of the os odontoideum. Only one-third of the patients had any signs or symptoms indicative of neural deficits, although most patients had cervical pain. Surgical treatment alleviated the pain and instability.