Microscopic changes in the thyroids of 68 patients who had received low-dose childhood irradiation to the head and neck and who presented with palpable thyroid abnormalities culminating in surgery are compared to 34 control thyroids obtained from age- and sex-matched autopsy cases. Eighty-eight percent of irradiated thyroids showed moderate to severe focal hyperplasia, 51% contained single or multiple adenomas or adenomatous hyperplastic nodules, 68% exhibited chronic lymphocytic thyroiditis, 51% revealed colloid nodules, 42% presented with oxyphile change, 25% had mild fibrosis and 59% contained well-differentiated papillary, follicular or mixed thyroid carcinoma averaging 1.6 cm in diameter. Three small carcinomas were of the sclerosing type. The non irradiated thyroids showed 32% colloid nodule formation, 17% focal hyperplasia, 6% adenomatous hyperplasia and no identifiable carcinomas. Several nonspecific histologic abnormalities are now recognized as following low-dose radiation to the thyroid, the most important being focal hyperplasia, which may represent a pre-malignant change in thyroid parenchyma.