Review of 137 patients with differentiated thryroid carcinoma (82 papillary and 55 follicular) demonstrates that external radiation in moderate dosage eradicates microscopic disease. Its use could lead to a decrease in surgical complications by avoiding unnecessarily radical attempts at removing all potential microscopic disease. Gross tumor also responds favorably to external radiation, but its very slow regression rate has led to the misconception that external radiation is ineffective in the treatment of these cancers. Both radioiodine and thyroid hormone play an important role in the management of differentiated thyroid cancers, but their effectiveness should not be overestimated.