Thyroid function was measured in 119 children, 16 years of age or less, after radiotherapy (XRT) for Hodgkin's disease. Thyroid abnormalities developed in 4 of 24 children (17%) who received 2600 rad or less, and in 74 of 95 children (78%) who received greater than 2600 rad to the cervical area, including the thyroid. The abnormality in all but three (one with hyperthyroidism and two with thyroid nodules) included the development of elevated levels of thyroid stimulating hormone (TSH). Age, sex, and administration of chemotherapy were not significant factors in the development of thyroid dysfunction. All children had lymphangiograms (LAG) and no time relationship was noted between thyroid dysfunction and LAG-XRT interval. The mean interval from radiotherapy to documented thyroid dysfunction was 18 months in the low-dose group and 31 months in the high-dose group, with most patients becoming abnormal within 3 to 5 years. Of interest was a spontaneous return of TSH to within normal limits in 20 children and substantial improvement in another 7. This study confirms the occurrence of dose-related occult hypothyroidism in children following external irradiation of the neck.