Three cases of diabetic ketoacidosis precipitated by thyrotoxicosis are presented. Two of them are young women with type 1 diabetes mellitus; the third case is a middle-aged woman with type 2 diabetes mellitus. All of them were diagnosed with Graves' disease. They typically showed tachycardia at rest in spite of correction of the metabolic disorder. Hyperthyroidism worsens glycemic control in diabetic patients and may precipitate diabetic ketoacidosis. On the other hand, women with diabetes have a higher prevalence of Graves' disease. Thus, in diabetic ketoacidosis without an obvious triggering factor, the presence of hyperthyroidism should be investigated, particularly in women.