An insulin-dependent diabetic patient received a renal transplant from a living related donor without evidence of rejection. In the posttransplant period, his serum potassium concentration (3.7 to 6.7 mEq/liter) fluctuated widely with the serum glucose concentration (165 to 470 mg/dl) during the day. Serum glucose and potassium concentrations were directly correlated (r = .734, P less than .001). Other factors controlling the serum potassium concentration were examined. Plasma and urinary aldosterone levels were normal, plasma renin activity and aldosterone levels rose during upright activity, and urinary potassium excretion increased with the administration of exogenous mineralocorticoid. Thus, mineralocorticoid secretion and responsiveness were intact. These observations indicate that hyperkalemia in a diabetic patient can occur in the absence of a defect in potassium excretion and are consistent with the interpretation that insulinopenia, as evidenced by hyperglycemia, can result in hyperkalemia due to diminshed translocation of both potassium and glucose from the extracellular to the intracellular compartment.