ABSTRACT The incidence of juvenile thyrotoxicosis has not exceeded one case per 100,000 population per year in Hungary during the past decades. From more than 200 simultaneous determinations of serum thyroid hormones (T3, T4, FT4) it was concluded that increased FT4 concentrations are the most frequent findings in thyrotoxicosis, even when both or one of the other thyroid hormone concentrations (T3, T4) are normal,-TSH and prolactin responses to TRH were studied in 18 patients; TSH remained always undetectable whereas prolactin increased in half of the patients. A significant negative relationship was found between the prolactin response and the level of circulating thyroid hormone. Although the pituitary lactotrophin is controlled by thyroid hormones, thyrotrophin is more sensitive to thyroid hormone levels in juvenile thyrotoxicosis.-In 56 TRH tests during antithyroid drug therapy this test was not found to be useful in predicting the likelihood of long-term remission. - The low maintenance dose of antithyroid treatment (2.5-5.0 mg/day) without thyroid hormone addition seems to be a favourable mode of therapy in juvenile thyrotoxicosis.