Thyroid function was measured serially in 28 children with Hodgkin disease diagnosed from 1971 to 1978. The patients' ages ranged from 4 to 16 years at diagnosis, and treatment consisted of chemotherapy only (four patients), radiation alone (15), or radiation plus chemotherapy (nine). None of the four children given chemotherapy only developed thyroid hypofunction, in contrast to 21 (88%) of the 24 children given high doses of radiation (P<0.001). Thyroid function in three patients with compensated hypothyroidism and in one child with primary hypothyroidism reverted to normal without thyroid replacement. One child given chemotherapy only and one child given radiation only became transiently hyperthyroid. These results indicate that patients given combined modality therapy for Hodgkin disease are at high risk for thyroid abnormalities. The results of long-term follow-up of thyroid function demonstrate, however, that all such thyroid abnormalities may not necessarily be permanent.