Observations available for patients with acute heart failure (HF) show conflicting results, and the prognostic role of anaemia ascertained on hospital admission is not well defined. We investigated the database of the Italian Survey on Acute Heart Failure (IS-AHF) to analyze prevalence, factors associated with and the prognostic role of anaemia (defined as haemoglobin < 12 g/dl) in patients hospitalized for acute HF with either depressed or preserved (>40%) ejection fraction (EF). The median haemoglobin level of the 2,318 patients considered in this analysis was 13 g/dl (inter-quartile range 11.5-14.3). The prevalence of anaemia was 31%. Patients who had anaemia were older, more frequently female gender, hospitalized for a chronic destabilized HF, had higher prevalence of preserved EF, hyponatremia, elevated troponin and other comorbidities (including diabetes, peripheral artery disease, chronic renal failure) than those who did not have anaemia. During the hospital stay, they were treated with higher doses of diuretics, and more frequently required mechanical ventilation and ultrafiltration, and less frequently received ACEi/ARB, aldosterone blockers and beta-blockers at hospital discharge. In-hospital mortality was 12.1 and 5.3% in patients with and without anaemia, respectively (p < 0.0001). In the multivariable analysis, anaemia was a significant independent predictor of in-hospital mortality apart from age, low systolic blood pressure, impaired renal function, elevated troponin assay, the non use of beta-blocker and the requirement of inotropic drug. In conclusion, anaemia diagnosed at hospital admission for acute HF is a frequent comorbidity with meaningful implications on the clinical management and prognosis both in patients with reduced and preserved EF.