Hypothyroidism was treated with desiccated thyroid substance or a rapid acting preparation in nine patients aged 39 to 84 years. The results are presented in tabular form. The treatment was generally effective, but four patients developed cardiac complications which are described in greater detail. Two patients died. In one patient, a woman 69 years old, it appeared that dosages of thyroid substance sufficient to remedy the myxedema could not be given without danger to the enfeebled heart. Intolerance to thyroid medication was most marked in older patients with a history and physical findings of arteriosclerotic heart disease. By commencing with small doses, two patients were enabled to take larger doses which had previously caused angina pectoris and congestive heart failure. Treatment of hypothyroidism in patients of this type should begin with doses of as little as 7.5 mg. of desiccated thyroid daily, with increments of 7.5 mg. every two or three weeks until a dose of 30 mg. a day is reached.