Potassium homeostasis was evaluated in 13 patients with diabetes mellitus. In eight, plasma renin activity was low; plasma aldosterone concentration was decreased in all; seven had a history of spontaneous hyperkalemia. After administration of glucose orally, there were paradoxical increases in serum potassium levels in seven patients. After potassium loading, maximal values and increments of serum potassium were higher and fractional potassium excretion was lower in the diabetic than in the control subjects, although the differences were not statistically significant. Abnormalities of potassium homeostasis in diabetes are probably related to insulin and mineralocorticoid deficiency. Diabetic patients with hypoaldosteronism have the potential for severe hyperkalemia should renal or extrarenal mechanisms for potassium homeostasis be challenged by severe acidosis be challenged by severe acidosis, diminished renal function, marked hyperglycemia, or administration of potassium salts or potassium-sparing diuretics.