Thirty children and adolescents with the clinical diagnosis of juvenile autoimmune thyroiditis (JAT) were studied. Twenty-six patients were female and four were male. The median age at diagnosis was 12.9 years (range, 7.5 to 17.3 years). At the initial evaluation, endocrine studies revealed that 40% of the patients were euthyroid, while 33% had compensated hypothyroidism and 27% had overt hypothyroidism. Seventeen patients, 16 of whom received thyroid hormone replacement therapy for a median period of 26 months, were followed up on a long-term basis after discontinuation of therapy. Seven (87%) of eight patients who were initially euthyroid have remained so for a median period of 28 months after treatment and a median period of 50 months since diagnosis. Four (44%) of nine patients who initially had hypothyroidism (either compensated or overt) have reverted to a euthyroid state and have remained so for a median period of 33 months after discontinuation of therapy. Neither the size of the thyroid gland nor the pattern or absolute titer of thyroid autoantibodies correlated with functional status. These results indicate that individuals with euthyroid JAT may remain euthyroid for many years and that a substantial percentage of patients with hypothyroid JAT will experience resolution of thyroid dysfunction.