Eleven patients treated for nasopharyngeal carcinoma with standard radiation therapy were found to have depressed cell mediated immunity. Post-treatment their total lymphocyte count was decreased by 60 per cent. Eight of 11 patients had depressed T-cell rosettes, and 9 of 10 had abnormal lymphocyte response to PHA. Immunosuppression was probably related to irradiation of large blood volumes, irradiation of the thymus, and malnutrition.