One hundred simultaneous determinations of serum T4 and T3 were performed during the course of treatment of 23 children and adolescents with Graves' disease. Five patients were previously untreated and six were experiencing relapse after treatment was discontinued. During relapse, increased T3 concentration is frequently present when serum T4 concentration is normal and T3 measurement is therefore more reliable for early detection of relapse. During therapy with thionamides, T4 measurement alone is often misleading in assessing adequacy of control achieved by therapy. Commonly, patients who clinically have hyperthyroidism have serum T4 concentrations within the normal range but continue to have elevated T3 concentrations (T3 toxicosis). Similarly, T4 can be suppressed into the hypothyroid range in the clinically euthyroid patient with either a normal or high T3 concentration. In these patients, determination of serum T3 often prevents premature reduction of thionamide dosage. When T4 and T3 concentrations do not clearly demonstrate presence or absence of hypothyroidism, measurement of serum thyroid stimulating hormone can be of value.