Among 1324 patients with well differentiated (papillary, follicular, Hurthle cell) thyroid carcinoma treated at the University of Texas M. D. Anderson Hospital and Tumor Institute between January 1950 and July 1984, 125 had a history of external irradiation to the head and neck region during childhood. This study was a comparison of the characteristics and disease course of thyroid carcinoma in these patients with those of patients who had not received such irradiation. Each irradiated patient was matched to 3 nonirradiated patients by age, sex, and extent of disease. The groups had similar distributions of histological type of lesion, surgical procedures, and number of patients who received radioactive iodine as part of their initial treatment. The two groups' recurrence and mortality rates were similar as well, although patients who had head and neck irradiation as children more often presented with thyroid cancer not limited to the thyroid gland and bilateral thyroid lobe involvement (P less than 0.005). The data indicate that patients who received head and neck irradiation have, at the time of diagnosis, more extensive thyroid carcinoma. However, the prognosis of well differentiated thyroid carcinoma in patients who have a history of head and neck irradiation is similar to that in patients without such a history.