BACKGROUND: Previous studies have associated reduced hemoglobin levels with increased adverse events in heart failure. It is unclear, however, whether this relation is explained by underlying kidney disease, treatment differences, or associated comorbidity. METHODS AND RESULTS: We examined the associations between hemoglobin level, kidney function, and risks of death and hospitalization in persons with chronic heart failure between 1996 and 2002 within a large, integrated, healthcare delivery system in northern California. Longitudinal outpatient hemoglobin and creatinine levels and clinical and treatment characteristics were obtained from health plan records. Glomerular filtration rate (GFR; mL.min(-1).1.73 m(-2)) was estimated from the Modification of Diet in Renal Disease equation. Mortality data were obtained from state death files; heart failure admissions were identified by primary discharge diagnoses. Among 59,772 adults with heart failure, the mean age was 72 years and 46% were women. Compared with that for hemoglobin levels of 13.0 to 13.9 g/dL, the multivariable-adjusted risk of death increased with lower hemoglobin levels: an adjusted hazard ratio (HR) of 1.16 and 95% confidence interval (CI) of 1.11 to 1.21 for hemoglobin levels of 12.0 to 12.9 g/dL; HR, 1.50 and 95% CI, 1.44 to 1.57 for 11.0 to 11.9 g/dL; HR, 1.89 and 95% CI, 1.80 to 1.98 for 10.0 to 10.9; HR, 2.31 and 95% CI, 2.18 to 2.45 for 9.0 to 9.9; and HR, 3.48 and 95% CI, 3.25 to 3.73 for <9.0 g/dL. Hemoglobin levels > or = 17.0 g/dL were associated with an increased risk of death (adjusted HR, 1.42; 95% CI, 1.24 to 1.63). Compared with those with a GFR > or = 60 mL . min(-1).1.73 m(-2), persons with a GFR <45 mL.min(-1).1.73 m(-2) had an increased mortality risk: adjusted HR, 1.39 and 95% CI, 1.34 to 1.44 for 30 to 44; HR, 2.28 and 95% CI, 2.19 to 2.39 for 15 to 29; HR, 3.26 and 95% CI, 3.05 to 3.49 for <15; and HR, 2.44 and 95% CI, 2.28 to 2.61 for those on dialysis. Relations were similar for the risk of hospitalization. The findings did not differ among patients with preserved or reduced systolic function, and hemoglobin level was an independent predictor of outcomes at all levels of kidney function. CONCLUSIONS: Very high (> or = 17 g/dL) or reduced (<13 g/dL) hemoglobin levels and chronic kidney disease independently predict substantially increased risks of death and hospitalization in heart failure, regardless of the level of systolic function. Randomized trials are needed to evaluate whether raising hemoglobin levels can improve outcomes in chronic heart failure.