Mild clinical hypothyroidism associated with low levels of serum total thyroxine T4) and tri-iodothyronine (T3) and raised levels of serum thyroid-stimulating hormone (T.S.H.) has been observed in 14 of 40 patients (35%) in the early months after a subtotal thyroidectomy for thyrotoxicosis under cover of propranolol. In 10 of the patients, however, the hypothyroidism was temporary and at 6 months after operation the thyroid hormone levels were normal and the serum T.S.H. levels had fallen. In 4 of the patients in whom clinical and biochemical evidence of hypothyroidism persisted 6 months postoperatively, longterm T4 replacement therapy was instituted. It is concluded that the diagnosis of permanent hypothyroidism should not be made with confidence before 6 months have elapsed after operation and that the incidence of hypothyroidism following the surgical treatment of thyrotoxicosis may have been overestimated in the past.