We investigated the importance of glucagon in the development of diabetic ketoacidosis by withholding insulin from six patients with juveniletype diabetes and four totally pancreatectomized subjects. Patients were fasting and had previously been maintained on intravenous insulin for 24 hours. In diabetic patients plasma glucagon concentrations rose sharply after withdrawal of insulin, and the increases were accompanied by a rise in blood ketone concentration of 4.1 ±0.7 (S.E.M.) and blood glucose concentration of 12.5±1.8 mmol per liter by 12 hours. In the pancreatectomized patients, despite the absence of measurable glucagon, blood ketones rose by 1.8±0.8, and blood glucose by 7.7±1.5 mmol per liter. Thus, glucagon is not essential for the development of ketoacidosis in diabetes, as has previously been suggested, but it may accelerate the onset of ketonemia and hyperglycemia in situations of insulin deficiency. (N Engl J Med 296:1250–1253, 1977)