Aims The prognostic significance of atrial fibrillation (AF) in hospitalized patients with heart failure (HF) remains poorly understood. To evaluate in what way AF and its different modes of presentation affect the in-hospital mortality in patients admitted with HF. Methods and results The EuroHeart Failure Survey was conducted to ascertain how hospitalized HF patients are managed in Europe. The survey enrolled patients over a 6-week period in 115 hospitals from 24 countries. For this analysis, patients were categorized into three groups according to the type of AF, previous AF (patients known to have had AF prior to admission), new-onset AF (no previous AF with AF diagnosed during hospitalization), and no AF (no previous AF and no AF during hospitalization). Clinical variables, duration of hospitalization, and in-hospital survival status were assessed and compared among groups. Of the 10 701 patients included in the survey; 6027 (57%) had no AF, 3673 (34%) had previous AF, and 1001 (9%) had new-onset AF. Patients with new-onset AF had a longer stay in the intensive care unit (ICU) when compared with previous AF and no AF patients (mean 2.6 +/- 5.3, 1.2 +/- 3.5, and 1.5 +/- 4.1 days, respectively; P < 0.001). In-hospital mortality was higher among patients with new-onset AF when compared with previous AF or no AF patients (12, 7, and 7% respectively; P < 0.001). After adjusting for multiple clinical variables, new-onset AF (not previous AF) was an independent predictor of in-hospital mortality (odds ratio 1.53, 95% CI 1.1-2.0). Conclusion In hospitalized patients with HF, new-onset AF is an independent predictor of in-hospital mortality and a longer ICU and hospital stay.