Background-Hyponatremia is a common electrolyte abnormality among patients hospitalized with heart failure and it is a marker for increased short-term and long-term mortality.However,little is known about the time course of hyponatremia and whether changes in serum sodium levels affect clinical outcomes.Methods and Results-Patients(n = 322)hospitalized with decompensated heart failure and serum sodium <135 mmol/L were evaluated.After hospital discharge,the first sodium value obtained within a 60-to 270-day period was recorded,and patients were classified into 3 groups,based on whether the serum sodium value increased(≥2 mmol/L),decreased(≤2 mmol/L),or remained unchanged(±1 mmol/L)relative to the baseline value.Kaplan-Meier survival curves were constructed to illustrate mortality as a function of change in sodium concentration over time,and a Cox-proportional hazards model was constructed to determine if change in serum sodium concentration predicted mortality after adjusting for relevant covariates.The mean age of the population was 66 years,45% were women,and 55% were white.The mean baseline sodium level was 131 mmol/L and the mean ejection fraction was 32.5%.Two hundred twenty-two patients(68.9%)exhibited an increase in sodium during follow-up;in 57 patients(17.7%)the level was unchanged and in 43 patients(13.4%)there was a decrease in sodium level.During a median follow-up of 610 days,there was a strong positive association between change in sodium level and survival(P for trend <0.001);that is,increased sodium was associated with decreased mortality.In multivariable analysis,change in sodium concentration and higher blood urea nitrogen were the strongest predictors of mortality(both P<0.0001).Conclusions-Among patients hospitalized with heart failure and hyponatremia,change in serum sodium concentration over time is a strong predictor of long-term survival. Background-Hyponatremia is a common electrolyte abnormality among patients hospitalized with heart failure and it is a marker for increased short-term and long-term mortality. Host, little is known about the time course of hyponatremia and whether changes in serum sodium levels affect clinical outcomes.Methods and Results-Patients (n = 322) hospitalized with decompensated heart failure and serum sodium <135 mmol / L were evaluated. After discharge of the first sodium value within a 60-to-270-day period was recorded, and patients were classified into 3 groups based on whether the serum sodium value increased (≥2 mmol / L), decreased (≤2 mmol / L), or remained unchanged (± 1 mmol / L) relative to the baseline value. Kaplan- Meier survival curves were constructed to illustrate mortality as a function of change in sodium concentration over time, and a Cox-proportional hazards model was constructed to determine if change in serum sodium concentration predicted mortality after adjusting for r Elevant covariates. The mean age of the population was 66 years, 45% were women, and 55% were white. The mean baseline sodium level was 131 mmol / L and the mean ejection fraction was 32.5%. Two hundred twenty-two patients ( 68.9%) showed an increase in sodium during follow-up; in 57 patients (17.7%) the level was unchanged and in 43 patients (13.4%) there was a decrease in sodium level. there was a strong positive association between change in sodium level and survival (P for trend <0.001); that is, increased sodium was associated with decreased mortality. In multivariable analysis, change in sodium concentration and higher blood urea nitrogen were the strongest predictors of mortality (both P <0.0001) .Conclusions-Among patients hospitalized with heart failure and hyponatremia, change in serum sodium concentration over time is a strong predictor of long-term survival.