Tetraparesis due to a spontaneous fracture of the base of the odontoid process occurred in a patient with undiagnosed, advanced ankylosing spondylitis. Few cases of insufficiency fractures of the proximal cervical spine in patients with loss of spinal mobility have been reported in the literature. The pathologic lesions that can cause severe neurologic compromise are reviewed. Because functional impairment and neurologic loss are common even after surgical decompression and stabilization, these lesions should be looked for during a careful physical evaluation followed by appropriate investigations.