Ten of 502 patients presenting with thyroid cancer were hyperthyroid due to Graves' disease (4 patients), multinodular goitre (3), an autonomous functioning nodule (1) and a large functioning tumour (2). In addition eight patients had a past history of Graves' disease and four of hyperthyroidism associated with multinodular goitre. Mortality in patients with Graves' disease and with multinodular goitre appeared similar to that of other patients of comparable age. Both patients with large functioning tumours died from progressive disease. Concentration 131I by tumour metastases was present in one patient with active Graves' disease who had a high serum concentration of TSH-receptor binding antibodies, indicating that these antibodies may chronically stimulate tumour function. The potential for 131I concentration by tumour when TSH secretion is suppressed should therefore be determined in patients with Graves' disease and if demonstrable tumour function is present, reflecting stimulation by Graves' immunoglobulins, then elimination of tumour remnants is particularly important.