Two patients with diabetes mellitus are described in whom Addison's disease and myxedema developed, in one instance 3 and in the other 16 years after the onset of the diabetes. Evidence is presented that both the hypothyroidism and hypoadrenocorticism in these patients were primary failures of the end-organ. In a third case, a sibling of one of the others, adrenal insufficiency preceded diabetes mellitus by 15 years. In the 2 cases tested, antibodies against the adrenal were demonstrable. One also had antithyroid antibodies, and the other antibodies against the parathyroid, in spite of clinically normal parathyroid function.