Two main patterns of hyperplasia of the thyroid epithelium are seen in hyperthyroidism--toxic diffuse hyperplasia and toxic nodular hyperplasia. Toxic diffuse hyperplasia may affect the entire gland or it may arise in the extranodular tissue in a gland with nodular lesions. A series of patients treated by subtotal thyroidectomy for hyperthyroidism were classified into three morphological groups of goitre: 98 had a toxic diffuse goitre (TDG), 58 toxic nodular goiter with diffuse hyperplasia (TNGDH), and 105 toxic nodular goitre with nodular hyperplasia (TNGNH). The type of goitre was correlated to the development of disturbed thyroid function 1-6 years after surgery. Twenty-two patients with TDG and 18 (31 per cent) with TNGDH developed hypothyroidism; only 3 (3 per cent) with TNGNH showed hypofunction. Six patients with recurrent hyperthyroidism were found; all showed diffuse hyperplasia of the gland. It is concluded that the development of disturbed thyroid function after surgery is closely related to the histopathological type of goitre.