In a recall clinic for patients at risk for thyroid carcinoma due to a history of radiation in infancy or childhood, a group of patients were randomly offered prospective suppressive L-thyroxine therapy and matched to a radiated nontreated group. With an average of more than two years' follow-up, the thyroid hormone-treated radiated group developed fewer nodules and abnormalities to palpation and also had a statistically significant lessening of minimal palpable abnormalities compared to the nonthyroxine-treated radiated group.