Postoperative thyroid status was classified into 6 categories on the basis of serum free thyroxine index (FTI), serum triiodothyronine (T-3) and serum TSH concentration. Review of 325 patients who underwent thyroidectomy for Graves' disease more than 4 years previously showed that 25 patients (7.7 per cent) had recurrent hyperthyroidism. Six patients (1.8 per cent) were classified as equivocal hyperthyroid since either FTI or T-3 was above the normal range. Two hundred and twenty-two patients (68.3 per cent) were unequivocally euthyroid and 33 patients (10.3 per cent) were euthyroid with elevated TSH levels. Twenty patients (6.2 per cent) were equivocal hypothyroid since either their FTI or T-3 values were below the normal range and TSH were increased. Hypothyroidism was present in 19 patients (5.8 per cent), of whom 11 had no clinical manifestation of thyroid dysfunction. The incidence of hypothyroidism did not correlate with the intervals between operation and review. The second review, performed in 189 patients 18 months after the first, showed that there was a change in thyroid functional status in 46 patients, of whom 32 had an increased level of function, including one hypothyroid and 7 equivocal hypothyroid patients who became euthyroid spontaneously. Thus postoperative hypothyroidism in some patients can recover without medication. Also thyroid function in some postoperative patients is not maintained at a fixed level but may fluctuate.