A familial asymptomatic os odontoideum with a Klippel-Feil type II fusion of C-2 and C-3 is reported. The pattern of inheritance within this family is consistent with that of autosomal dominance. The index case, a 16-year-old boy, was studied with plain cervical spine x-ray films, lateral cervical tomography in flexion and extension, fluoroscopic evaluation of the subluxation, and magnetic resonance (MR) imaging of the spine in flexion and extension. In spite of the subluxation noted on flexion and extension, there was no evidence of cord compression on MR imaging. The etiology and management of this condition are discussed.