AIMS: Red blood cell distribution width (RDW) predicts mortality in chronic heart failure (HF) and stable coronary artery disease. The prognostic value of RDW in more acute settings such as acute HF, and its relative prognostic value compared with more established measures such as N-terminal pro-brain natriuretic peptide (NT-proBNP), remains unknown. METHODS AND RESULTS: In a cohort of 205 patients with acute HF, independent predictors of RDW were identified using linear regression analysis. The association between RDW and 1-year survival in the context of other predictors was assessed using Cox's proportional hazards analysis. Red blood cell distribution width was elevated in 67 (32.7%) patients; RDW was independently associated with haematological variables such as haemoglobin (P < 0.001) as well as the use of loop diuretics (P = 0.006) and beta-blockers (P = 0.015) on presentation, but not with nutritional deficiencies, recent transfusion, or inflammatory variables. Log-transformed RDW values independently predicted mortality in multivariable Cox's proportional hazards analysis (hazards ratio, 1.03; 95% confidence interval, 1.00-1.06; P = 0.04); when stratified on the basis of RDW and NT-proBNP status, the combination provided additional prognostic information. CONCLUSION: Red blood cell distribution width is frequently elevated among patients with acute HF and does not appear to be associated with nutritional status, transfusion history, or inflammation. Red blood cell distribution width independently predicts 1-year mortality in acute HF. The value of RDW appears additive to other established prognostic variables such as NT-proBNP.