There have been several reports1 of the use of thiouracil in the treatment of thyrotoxicosis since Astwood's2 important discovery of the value of this drug, but most of these have dealt with only small series of cases. Now, however, we report on the use of this drug in the treatment of 152 patients with hyperthyroidism. The age of the patients varied from 5 to 72 years, and the thyrotoxicosis had existed for from three weeks to twenty-two years. One or more subtotal thyroidectomies had been performed in 17, and 29 had taken iodide until less than a month before thiouracil therapy was begun. In 7 instances iodide therapy had been discontinued because of the development of an associated rash. While most of the patients had diffusely hypertrophied thyroid glands, many had nodular goiters. In 23 cases there were clearcut symptoms of myocardial insufficiency with cardiac dilatation and hypertrophy,