From March 1980 to August 1982, 108 patients with Graves' disease participated in a prospective study at Yonsei University College of Medicine. Preoperatively, 22 patients were prepared with propranolol alone (group 1) and 86 with propranolol and propylthiouracil (PTU) (group 2). The duration of preparation averaged 11.5 days for group 1 and 10.8 days for group 2. Bilateral subtotal thyroidectomy was performed, leaving 4 to 6 grams at both upper poles. No significant difference was noted in the average weight of the removed gland or the amount of blood loss between both groups. During the operation, the pulse rate of group 1 was significantly faster than that of group 2 but evened out by the day of discharge. The incidence of high fever during and immediately after operation was 27.3 per cent in group 1 which is greater than the 17.4 per cent in group 2. In group 1, the incidence of postoperative transient hypocalcemia was 27.3 per cent, transient or prolonged hypothyroidism was 13.6 per cent and recurrence of thyrotoxicosis was 4.5 per cent. In group 2, the values were 22.1, 18.6 and 4.7 per cent, respectively. High fever developed in one patient from group 1 during operation and this patient died on the third day postoperatively. From the results of our study, it appears to be more beneficial to administer both propranolol and PTU as preoperative medication to patients with Graves' disease prior to undergoing operation.