Thyroid evaluations were performed in 95 patients who received radiotherapy to the neck region for childhood cancer five to 34 years earlier. Fifty-six patients (61 percent) had at least one abnormality of serum free thyroxine index, serum thyroid-stimulating hormone (thyrotropin), or thyroid palpation. Seven had subnormal free thyroxine index and 40 had elevated thyrotropin concentrations. Thyroidal radiation doses of 3,000 or more rads and lymphangiography independently increased the risk (p less than or equal to 0.01) of an elevated serum thyrotropin concentration (present in 11 percent of patients with neither risk factor, 50 percent of those who underwent lymphangiography and received less than 3,000 rads, 46 percent of those who had 3,000 or more rads and no lymphangiography, and 76 percent of those with both), but duration of follow-up did not. Twenty-six patients had thyroid nodules and six others had diffuse thyroid enlargement. The frequency of palpable abnormalities increased with the follow-up time after radiation (30 percent of patients followed up less than 10 years had abnormalities versus 43 percent of those followed up 10 or more years, p = 0.03), but was not related to the serum thyrotropin level, radiation dose, or lymphangiography. Among 10 patients who had surgery for nodules, three had localized papillary thyroid carcinomas.