METHODS A case report of a patient in whom atlantoaxial instability developed secondary to repeat radiation therapy for recurrent nasopharyngeal carcinoma. OBJECTIVE To illustrate a dramatic and previously unreported complication of local radiation to the posterior nasopharynx. BACKGROUND Nasopharyngeal carcinoma is an unusual tumor that usually is managed with local, external-beam radiation. It is not thought to involve the cervical spine directly, although local invasion of the skull base is common. METHODS A review of the medical records and radiographs of the only patient known to develop this complication of radiation used to manage nasopharyngeal carcinoma. RESULTS Atlantoaxial instability developed in a patient as a result of repeat radiation for a locally recurrent tumor. The instability was associated with intrusion of the anterior arch of C1 into the posterior nasopharynx and was managed successfully with a posterior stabilization using transarticular screws and supplemental wiring. CONCLUSIONS Patients who have undergone local irradiation for nasopharyngeal carcinoma may be at risk for developing atlantoaxial instability.