A 44-year-old woman was admitted because of stupor. She had consumed 3 liters of water due to thirst after drinking alcohol. Laboratory findings on admission revealed marked hyponatremia (sodium: 115 mEq/l). She was diagnosed as having water intoxication. She recovered from her hyponatremia upon excretion of a large amount of hypotonic urine. Subsequently, however, her serum creatine phosphokinase was markedly elevated at 28,650 IU/l, and her serum myoglobin reached 2,760 ng/ml. The relationship between the occurrence of hyponatremia secondary to water intoxication and rhabdomyolysis was suggested.