Between 1973 and 1980, 93 patients with Graves' disease underwent subtotal thyroidectomy by the same surgeon (the size of thyroid remnant was 4 g per side). No case of operative mortality, no case of thyroid storm nor of surgical complications occurred. Three months after surgery 40% of patients were euthyroid, 25% had overt hypothyroidism, 35% had subclinical hypothyroidism. In the following yr important variations of thyroid function were observed. The number of patients with subclinical hypothyroidism decreased slowly (22% and 9% at 3 and 6 yr, respectively), and some became euthyroid, some hypothyroid, others relapsed. Seven patients had recurrent hyperthyroidism. In particular at 3 yr 45% of patients were euthyroid, 28% had overt hypothyroidism, 22% had subclinical hypothyroidism, 4% had recurrence; at 6 yr 56% were euthyroid, 32% had overt hypothyroidism, 9% had subclinical hypothyroidism, 3% had recurrence. Four out of the 8 patients operated under 20-yr-old became hypothyroid in comparison with only 2 out of the 15 patients over 50-yr-old. Relapses were present only in patients operated at less than 40-yr and only in females. No correlation was found between thyroid lymphocytic infiltration and thyroid function after surgery, nor between the presence of antithyroid antibodies and hypothyroidism. All cases who relapsed had high TMA titers both before and after operation. This study confirms the need for accurate follow-up after subtotal thyroidectomy for Graves' disease.