Atrial fibrillation (AF) is the most common arrhythmia in patients with heart failure (HF); despite its negative hemodynamic consequences, the prognostic influence of AF in patients with HF remains controversial. Th e aims of the paper - To evaluate the atrial fibrillation prevalence in a population of patients hospitalized for heart failure with reduced ejection fraction (HFREF) who were in NYHA III and IV functional classes. To emphasize the clinical, epidemiological and treatment characteristics of patients with heart failure and reduced ejection fraction and AF and to compare them to those in sinus rhythm (SR); To evaluate the association between AF and general mortality in this group of patients. Materials and methods - It is a retrospective study that enrolled 882 consecutive hospitalized patients between 1.01.2004 and 31.10.2007 in the Cardiology I Department from "C.C. Iliescu" Institute of Emergency of Cardiovascular Diseases, having as main diagnosis HFREF in III and IV NYHA functional classes. Depending on the AF presence, the entire group has been divided in the subgroup with AF and in the subgroup with SR. Th e data analysis used descriptive statistics methods, Kaplan-Meier survival analysis, death rates comparison. Th e statistical significance of the p value threshold was below 0.05. Th e independent contribution of each of the associated factors with the out of hospital mortality when using the unvaried analysis was later appreciated through multivariate analysis using the regression model of the proportional risk (Cox). Th e mean follow-up period was approximately 30 months. Results - Th e AF was present in 31.5% of the patients at enrolment stage. Patients with HFREF were mostly men not only within the general group of patients (71.2% M vs 28.8% F), but also within the AF subgroup (77% M vs 23% F) and within SR subgroup respectively (68.5% M vs 31.5% F). Th e patients in AF, compared to those in SR, were older (65.6 years vs 62.8 years; p=0.002), had a higher percentage of alcohol consumers (15.1% vs 6.6%; p=0.002), had higher cardiac frequency (81.47 bpm vs 75.15 bpm; p<0.001), more dilated LA (55.04 mm vs 50.7 mm; p<0.001), higher levels of blood creatinine (1.85 mg/dl vs 1.20 mg/dl; p=0.004), the HF etiology was most frequent non-ischemic (62.9% vs 49.8%; p<0.001), and also had multiple co-morbidities. Th e general death rate of patients with HFREF at the end of the follow-up period was 41.5%. Th e specific death rate in patients with AF was significantly higher compared with those in SR (47.5% vs. 38.7%; p<0.05). Th e multivariate analysis emphasized the AF contribution to the increase of all-cause mortality (HR 1.257; 95% CI 1.012-1.560; p=0.03). In patients with left ventricle ejection fraction below 25%, the AF loses its function of prognostic independent factor for all-cause mortality. Conclusions - AF was proven to be an independent prognostic factor for the excess of mortality in patients with HF and moderate / severe systolic dysfunction. [ABSTRACT FROM AUTHOR]