Hyponatremia is known to be a poor prognostic factor in patients hospitalized with heart failure (HF), however not well studied in Japan. The aims of this study were to characterize hyponatremic hospitalized patients with HF and to clarify the relations between hyponatremia and detailed in-hospital outcomes in Japan. Among 4,837 hospitalized patients with HF enrolled in the Acute Decompensated Heart Failure Syndromes (ATTEND) registry, patient characteristics and in-hospital mortality in those with hyponatremia were examined. Hyponatremia (sodium <135 mEq/L) was observed in 11.6% of patients. Patients with hyponatremia were of similar age, included fewer men, and had a higher proportion of previous hospitalizations for HF compared to those with normonatremia. On admission, lower heart rates and blood pressures and higher brain natriuretic peptide levels were observed in patients with hyponatremia. During hospitalization, inotrope levels and mechanical device use were significantly higher in patients with hyponatremia. Rates of all-cause and cardiac death were significantly higher in patients with hyponatremia, 15.0% and 11.4%, respectively, compared to 5.3% and 3.6%, respectively, in those with normonatremia. In hyponatremic hospitalized patients with HF, cardiac death accounted for 76.2% of all-cause death. In conclusion, the present study demonstrates that in Japan hyponatremia in patients hospitalized with HF is relatively common and is associated with a very high in-hospital mortality. (C) 2013 Elsevier Inc. All rights reserved. (Am J Cardiol 2013;111:1019-1025)