Objectives. Although diabetes mellitus is frequently associated with heart failure (HF), the association between elevated admission glucose levels and adverse outcomes has not been well established in hospitalized patients with acute HF. Methods. We prospectively evaluated in-hospital mortality, post-discharge 1-year mortality and 1-year re-hospitalization rates in the Italian Network on Heart Failure(IN-HF) Outcome registry cohort of 1,776 patients hospitalized with acute HF and stratified by their admission glucose levels (i.e., known diabetes, newly diagnosed hyperglycemia, no diabetes). Results. Compared with those without diabetes (n=586), patients with either known diabetes (n=749) (unadjusted-odds ratio [OR] 1.64, 95%CI 0.99-2.70) or newly diagnosed hyperglycemia (n=441) (unadjusted-OR 2.34, 95%CI 1.39-3.94) had higher in-hospital mortality, but comparable post-discharge 1-year mortality rates. After adjustment for age, sex, systolic blood pressure, estimated glomerular filtration rate, left ventricular ejection fraction, HF etiology and HF worsening/de novo presentation, the results remained unchanged in patients with known diabetes (adjusted-OR 1.86, 95%CI 1.01-3.42), while achieved borderline significance in those with newly diagnosed hyperglycemia (adjusted-OR 1.81, 95%CI 0.95-3.45). One-year re-hospitalization rates were lower in patients with newly diagnosed hyperglycemia (adjusted-hazard ratio 0.74, 95%CI 0.56-0.96) than in other groups. Conclusions. Elevated admission blood glucose levels are associated with poorer in-hospital survival outcomes in patients with acute HF, especially in those with previously known diabetes. This finding further highlights the importance of tight glycemic control during hospital stay and address the need of dedicated intervention studies to identify customized clinical protocols to improve in-hospital survival of these high-risk patients.