Objectives: Heart failure (HF) is associated with high mortality and atrial fibrillation (AF) is the most common arrhythmia encountered in HFpatients with reduced ejection fraction (EF). We aimed to assess whether new-onset AF is an independent poor prognostic factor in this group ofpatients. We also searched for parameters that might influence the in-hospital mortality in reduced EF heart failure (HFrEF) patients.Materials and Methods: The study was a retrospective observational study investigating the admission characteristics and in-hospital events for119 HFrEF patients at sinus rhytym, admitted for decompensated heart failure. We evaluated the in-hospital mortality and aimed to identify thepredictive factors.Results: Overall 12% of the heart failure patients died during hospitalization. The mean age of the study population was 71±9 years with 37% female.The mean EF was 27±7% and mean duration of hospitalization was 9±4 days. Patients with lower body mass index, lower glomerular filtration rateand patients with longer hospitalization had significantly higher in-hospital mortality rates (p=0.02, p=0.04 and p=0.001, respectively). New-onset AF,restrictive filling pattern and being angiotensin-converting enzyme inhibitors/angiotensin receptor blockers naive were factors significantly relatedto higher mortality (p=0.001, p=0.001 and p=0.02, respectively). Long hospitalization duration and new-onset AF at hospital were independentpredictors of in-hospital mortality [p=0.006 Odds ratio (OR): 1.394 (1.098-1.771) and p=0.012 OR: 10.869 (2.677-71.428), respectively].Conclusion: In hospital outcome of patients admitted with decompensated HFrEF is poor. In our trial, patients with new-onset AF and patients withlonger hospitalization duration had higher mortality rates. An understanding of the risk factors for in-hospital deaths may help improving intensivecare for this patient population.