Description: Background Hyponatremia predicts poor prognosis in patients with acute heart failure (AHF). However, the association of the severity of hyponatremia and changes of serum sodium levels with long-term outcome has not been delineated. Methods and Results The study population was drawn from the HARVEST registry ( H eart F a ilure R egistry of Taipei Ve teran s General Hospi t al), so that patients hospitalized for acute heart failure (AHF) composed this study. The National Death Registry was linked to identify the clinical outcomes of all-cause mortality and cardiovascular death, with a follow-up duration of up to 4 years. Among a total of 2556 patients (76.4 years of age, 67% men), 360 had on-admission hyponatremia, defined as a serum sodium level of 〈135 mEq/L on the first day of hospitalization. On-admission hyponatremia was a predictor for all-cause mortality (hazard ratio and 95% CI: 1.43, 1.11–1.83) and cardiovascular mortality (1.50, 1.04–2.17), independent of age, sex, hematocrit, estimated glomerular filtration rate, left ventricular ejection fraction, and prescribed medications. Subjects with severe hyponatremia (〈125 mEq/L) would even have worse clinical outcomes. During hospitalization, a drop of sodium levels of 〉3 mEq/L was associated with a marked increase of mortality than those with minimal or no drop of sodium levels. In addition, subjects with on-admission hyponatremia and drops of serum sodium levels during hospitalization had an incremental risk of death (2.26, 1.36–3.74), relative to those with normonatremia at admission and no treatment-related drop of serum sodium level in the fully adjusted model. Conclusions On-admission hyponatremia is an independent predictor for long-term outcomes in patients hospitalized for AHF. Combined the on-admission hyponatremia with drops of serum sodium levels during hospitalization may make a better risk assessment in AHF patients.