The reported relationship of radiation exposure and thyroid carcinoma stimulated this retrospective study of 298 patients treated at St. Jude Children's Hospital with radiation therapy to the neck for childhood cancer to identify patients who developed subsequent thyroid abnormalities. This series includes 153 patients with Hodgkin's disease, 95 with acute lymphocytic leukemia, 28 with lymphoepithelioma, and 22 with miscellaneous tumors. Inclusion in the study required 5 years of disease-free survival following therapy for their original tumor, which included thyroid irradiation. Follow-up has been 100%. Most patients also received chemotherapy. Seventeen patients were found to have decreased thyroid reserve with normal levels of free triiodothyroxine (T3) or free thyroxin, (T4) and an elevated level of thyroid-stimulating hormone (TSH). In nine patients hypothyroidism developed, with decreased T3 or T4 levels and an elevated level of TSH. One hyperthyroid patient was identified. Two patients had thyroiditis, and seven had thyroid neoplasms: (carcinoma in two, adenoma in two, colloid nodule in one, and undiagnosed nodules in two). This survey has demonstrated an increased incidence of thyroid dysfunction and thyroid neoplasia when compared to the general population. The importance of long-term follow-up for thyroid disease is emphasized in patients who have received thyroid irradiation. The possible role of subclinical hypothyroidism with TSH elevation coupled with radiation damage to the thyroid gland as a model for the development of neoplastic disease is discussed.