One hundred patients who underwent bilateral subtotal thyroidectomy for Graves' disease between January 1980 and September 1984 have been evaluated. The observation period ranged from 5 to 9 years, the average being 6.2 years. Postoperative thyroid function was evaluated with T3, T4 and TSH and compared with their clinical manifestations. Eighty-two patients became euthyroid, 14 patients had recurrence and 4 patients developed hypothyroidism. The thyroid hormone level of euthyroid patients were in an unstable state up to 5 years after the operation. Sixteen variables which might influence the postoperative recurrence and hypothyroidism were analyzed but no statistically significant factors were determined, although recurrences were found frequently in patients over 30 years, the patients with lower infiltration of lymphocytes and absent of fibrosis of thyroid tissue. The results obtained in the present study suggest that mean 6.0 gm of remnant thyroid is suitable for maintaining euthyroidism postoperatively in a majority of patients. In addition, patients should be followed closely for many years and should undergo hormonal determination periodically because recurrence and hypothyroidism can occur at 5 years or more after the operation.