The case histories of four patients who developed hemiatrophy of the tongue from 3 to9 years after a course of curative radiation therapy for carcinomas of the head and neck are presented. These patients subsequently followed for from 1 1/2 to 6 years withoutlocal recurrence of the tumor, distant metastasis, or involvement of other cranial nerves, indicative of only a unilateral hypoglossal nerve atrophy. A review of the literatureshowed that peripheral and cranial nerve damages after radiation therapy have been reported for the optic nerve, hypoglossal nerve, oculomotor nerve, abducens nerve, recurrent laryngeal nerve, brachial plexus nerves and peripheral nerves of the extremeties. Reviewof clinical and experimental data indicated that in most cases, the damages were probably caused by extensive connective tissue fibrosis around and infiltrating the nerve trunks. Three possible types of peripheral and cranial nerve damages after radiation therapy are identified.