A 42-year-old man previously diagnosed with congenital diabetes insipidus (DI) developed polydipsia and polyuria (water intake and urine excretion, above 20 l per day). On admission, a serum sodium level of 124 mEq/l, a serum chloride level of 87 mEq/l, and plasma osmolality of 254 mOsm/kg H₂O were found, although urine osmolality was always less than 100 mOsm/kg H₂O, with daily urine excretion of more than 15 l. Creatinine clearance was 153 l/day. Initially, he had free access to water and his daily water intake was 20–27 l. Restriction of water intake (10 l/day) normalized his serum sodium levels and alleviated his general fatigue. Hyponatremia is a rare fluid electrolyte disturbance in DI. We found no reports of hyponatremia associated with congenital nephrogenic DI, presumably because the osmosensing mechanism is intact in such patients. The use of thiazides for the treatment of polyuria, in combination with potassium-sparing diuretics and salt restriction, may accelerate the genesis of hyponatremia caused by psychogenic polydipsia in nephrogenic DI.