A patient who had experienced water intoxication despite normal renal function and normal urinary diluting ability was observed during the initiation stage of hyponatremia. Upon the excessive water intake (10 to 15 L) for several days, he developed moderate hyponatremia (121 mEq/L) and headache, an early symptom frequently seen in water intoxication. During this period, his urine was maximally dilute (50 to 60 mOsm/kg H2O), and his urinary sodium excretion increased. This report suggests that 10 to 15 L of water intake for several days can cause water intoxication in subjects with normal urinary diluting ability and that the increase in sodium excretion is prerequisite for the high urine flow rate.