We demonstrated severe hyponatremia in a 68 year-old man who had pituitary tumor. He had poor appetite and was disoriented. Tests revealed hyponatremia of 110 mmol/l, and he was admitted to Jichi Medical School Omiya Medical Center to undergo further tests. Physical findings revealed disturbance of consciousness with Japan Coma Scale I-2. There was neither dehydration nor edema. Laboratory data showed a serum sodium level of 112 mmol/l; plasma osmolality, 219 mmol/kg; and urinary osmolality, 555 mmol/kg. Plasma arginine vasopressin (AVP) level was 1.6 pmol/l despite the marked hypoosmolality. Anterior pituitary function was normal. Brain magnetic resonance imaging showed a pituitary tumor of 20 × 18 × 20 mm in size, which pushed the pituitary stalk upward. After the adenomectomy, serum sodium level was kept normal without any treatment. Histology showed basophilic adenoma. These findings indicate that local pituitary tumor may cause exaggerated secretion of AVP, resulting in the syndrome of inappropriate secretion of antidiuretic hormone (SIADH).