In sixteen cases of toxic adenoma of the thyroid (autonomous hot nodule with complete suppression of the surrounding normal parenchyma) potassium iodide was given in doses of 100 microgram/day for one week, 200 microgram/day for another and 400 microgram/day for a third week. There was a progressive increase in the serum T4 level. Serum T3 also increased, although this was significant only after the first week. Serum TSH was undetectable throughout the entire period of the study. This metabolic pattern is different from the response seen in cases of nontoxic endemic goitre, where small iodine supplements induce an increase in serum T4 but a decrease in serum T3. Furthermore, the present results may explain the phenomenon of iodine-induced or iodine-precipitated hyperthyroidism (Jod-Basedow) when patients with autonomous thyroid are presented with a high iodine intake. In contrast to the results obtained with small iodide doses, two other cases treated with large pharmacological doses of iodide showed a decrease in both serum T4 and serum T3. It is concluded that the physician should be aware of the possibility of precipitating or aggravating thyrotoxicosis in patients with an autonomous hot nodule by increasing their intake of iodine.