Objectives The study sought to investigate the clinical correlates and prognostic role of anemia and changes in hemoglobin in patients hospitalized for acute decompensated heart failure (AHF). Background Anemia is related to a poor outcome in patients with heart failure. In addition, an increase in hemoglobin during hospitalization might be a sign of effective decongestion and therefore related to improved outcome. Methods This is a post hoc analysis of the PROTECT (Placebo-Controlled Randomized Study of the Selective Adenosine A1 Receptor Antagonist Rolofylline for Patients Hospitalized with Acute Decompensated Heart Failure and Volume Overload to Assess Treatment Effect on Congestion and Renal Function) study in 1,969 patients with AHF and mild to moderate impaired renal function. Hemoglobin levels were measured daily for the first 4 days and at day 7. The endpoint was 180-day all-cause mortality. Results Anemia at baseline was observed in 50.3% of the patients. During follow-up, 359 patients (18.2%) died. Hemoglobin increased in 69.1% and was associated with a better renal function at baseline and more weight loss, but was associated with a deterioration of renal function (p = 0.01), whereas total dose diuretics was lower in patients with hemoconcentration (p