Hemoconcentration is a surrogate marker of effective decongestion and diuresis therapy. Recently, hemoconcentration has been associated with decreased mortality and rehospitalization in heart failure (HF) patients. However, the prognostic power of hemoconcentration in a large sample-sized HF cohort was limited until now.We analyzed data from hospitalized patients with acute heart failure (AHF) that were enrolled in the Korean Heart Failure Registry(n=2,357). The primary end point was a composite of all-cause mortality and HF rehospitalization during the follow-up period (median=347, interquartile range=78-744 days).Hemoconcentration, defined as an increased hemoglobin level between admission and discharge, was presented in 1,016 AHF patients (43.1%). In multivariable logistic regression, hemoglobin, total cholesterol, and serum glucose levels at admission, and ischemic HF, were significant determinants for hemoconcentration occurrence. The Kaplan-Meier curve showed that event-free survival was significantly higher in the hemoconcentration group compared to the non-hemoconcentration group (65.1% vs. 58.1%, log rank p<0.001). In multiple Cox proportional hazard analysis, hemoconcentration was an independent predictor of the primary end point after adjusting for other HF risk factors (hazard ratio=0.671, 95% confidence interval=0.564-0.798, p<0.001).Hemoconcentration during hospitalization was a prognostic marker of fewer clinical events in the AHF cohort. Therefore, this novel surrogate marker will help in the risk stratification of AHF patients.