A study was made of 43 patients with heart disease who could not be restored to a comfortable state of ambulation by treatment with digitalis, diuretics, low-sodium diet, and other medication. In this group there were 5 men and 20 women with definite evidence of thyroid abnormalities. In none of the 25 was the hyperthyroidism diagnosed clinically, although it was looked for by both history and physical examination; the evidence was obtained in each case by the radioactive iodine (I131) tracer technique. The diagnostic and therapeutic use of I131 is exemplified in the history of a 53-year-old man with angina pectoris. It is believed that by careful adjustment of I131 dosage the patient can be brought to a state of normal rather than subnormal thyroid function. The procedure outlined gave marked improvement in all patients but two who died of myocardial infarction.