A 62-year-old man with colon cancer who presented with hyponatremia is described. Volume depletion, renal failure, and cardiac, adrenal, hepatic, and thyroid diseases were excluded as causes of hyponatremia. The urine sodium concentration was repeatedly increased, suggesting the presence of the syndrome of inappropriate antidiuretic hormone secretion. An intact urinary diluting ability and the ability to maintain sodium balance without correcting hyponatremia when the sodium intake was high were consistent with the diagnosis of the reset osmostat variant of the syndrome of inappropriate antidiuresis.