The effects of administration of iodine (1 mg/day orally, 64 days) were studied in three siblings with congenital goiter and hypothyroidism due to defective thyroglobulin (Tg) synthesis. The patients presented very large goiters, elevated RAI uptake, negative perchlorate discharge test, low serum T4, and elevated TSH concentrations. Immunoassayable Tg was low and failed to increase after stimulation with exogenous bovine TSH. Analysis of individual thyroid extracts by gel filtration failed to reveal a Tg component; the immunoassayable Tg antigens in these goitrous tissues were 0.12 and 0.21 mg/g tissue, respectively (normal 70-90 mg/g tissue). The histological pattern of their thyroids was compatible with defective Tg synthesis. The administration of iodine caused a rise in the mean serum T4, T3, and free T4 concentrations in all three siblings, but did not alter the serum Tg concentration. TSH concentrations rose in the terminal period of observation in the three subjects and this was considered to be due to a possible effect produced by the iodine load in the thyroperoxidase system (Wolff-Chaikoff effect). One patient showed an increase in goiter size during the period of observation. These results suggest that iodine administration enhanced the ability of the dyshormonogenetic gland to synthesize iodothyronines.