It is known that radiotherapy (Rx) has a dose-dependent deleterous effect on the thyroid gland. Primary hypothyroidism occurs in 20 to 70% of H and it becomes evident late, between 3-6 years post-Rx. In this work, thyroid function was evaluated in H (n=62, range of chronological age: 4.52-17.3 years). According to prognosis of H (GATLA score). 3 groups were defined: A)Favorable, n=14, B)intermediate, n=33, and C)disfavorable, n=15. Every group was subdivided according to time after the end of treatment in, ST (<6 months) and LT (>6, range 6 to 96 months). Methods: TRH-TSH test(μUl/ml), T4 (mcgr/dl), T3 (ng/ml), thyroid antibodies (Tab) by hemagglutination. Hypothyroidism was considered whenever an inequivocal alteration of TRH-TSH was detected (Basal TSH>5μlU/ml, and response>30 μlU/ml). Group 1 received only chemotherapy (Chem). Group B, Rx + Chem, and Group C. Rx + aggressive Chem. Rx dose was similar in B and C. Results: Incidence of hypothyroidism A)ST: 1/4, LT 2/10 (Fisher's exact test. ST vs LT p=ns; B)ST 10/15, LT 11/18 (p=ns); C)ST 1/7, LT 6/8(p=0.04). There was no significant difference between A, B or C in neither ST or LT in T4 and T3. Only 6/62 H had high titers of Tab. H ST there was significantive difference only in B vs C (p=0.02) while at LT, it was found in A vs B (p=0.05) and in A vs C (p=0.05). The difference between B and C at ST is probably related to immunological or genetic differences in the population and not to the dose of Rx which was similar in both groups. The frequency of hypothyroidism in Group A at ST and at LT (25%) is higher than in the juvenile population (2%) supporting the concept of genetic or immunological predisposing factors in H. A deficiency of suppressor T lymphocytes (T-S) in the pathogenesis of autoimmune thyroid disease has been shown. Furthermore, T-S are highly cooperative while T-helper lymphocytes, which cooperate with B lymphocytes in the production of antibodies are not resistant. On the other hand. Chem would induce damage in B lymphocyte clones being formed at the time, producing a definitive lesion in the gland and explaining the low Tab titers. The high incidence of hypothyroidism in Group C at LT is probably related to the known delayed effect of Rx in addition to the use of aggressive Chem which includes adriamicine, a thyrotoxic cytostatic.