Graves' disease and nodular toxic goitre are known to occur with increased frequency in patients with diabetes mellitus. It may be difficult to distinguish the symptoms of the two conditions from each other at the onset of diabetes. Thyrotoxic crisis is an uncommon, but extreme accentuation of thyrotoxicosis with a high mortality even in patients undergoing correct treatment. Reports on "major adverse life events" precipitating either diabetes or thyrotoxicosis are known. The symptoms of thyrotoxic disease in the elderly are often vague with few symptomatic manifestations. Five cases with simultaneous onset of insulin-dependent diabetes mellitus (IDDM) and thyrotoxicosis are presented: one in an adult patient with manifestations of thyrotoxic crisis, one in early adolescence, two in elderly patients with multinodular goitre and Graves' disease respectively, and finally in a younger IDDM patient thyrotoxicosis had initially been overlooked. The five cases leading to the suggestion that screening for hyperthyroidism in IDDM-patients by measuring TSH should be considered at the onset of diabetes and in dysregulated diabetic patients.