The one-step radioimmunoassay of free thyroxine (f-T<sub>4</sub>) using a gamma-coated kit was used in this study. In controls the mean plasma levels (pmol/l) were 20.8 (range 13.5–37) during the first days of life, 17.4 (11.5–27) from 1 week to 1 year, and 17.0 (10–24.5) after 1 year. In preterm newborns it was correlated with the length of gestation. Among 23 untreated hypothyroid infants aged 15–22 days, f-T<sub>4</sub> was undetectable in those with athyreosis, while in cases with dysgenetic thyroid it was variable, correlated to the width of the gland (r = 0.77, p < 0.01). In 44 hypothyroid patients treated with l-T<sub>4</sub>, a highly significant positive correlation was found after the 1 st month of treatment between plasma f-T<sub>4</sub> and the daily l-T<sub>4 </sub>dose (r = 0.46, p < 0.01), and a negative one between f-T<sub>4</sub> and plasma TSH (r = -0.59, p < 0.001). It is concluded that measurement of f-T<sub>4</sub> offers a valuable means for control of diagnosis and treatment in congenital hypothyroidism, especially useful for avoiding both under-and overtreatment. Its correlations suggest that it is the most reliable hormonal measurement in the follow-up of thyroid children.