Background:The prognostic impact of new-onset atrial fibrillation (AF) is not fully elucidated.Methods and Results:We examined 4,818 consecutive stage C/D chronic heart failure (CHF) patients in the Chronic Heart Failure Analysis and Registry in the Tohoku District-2 (CHART-2) Study (n=10,219). At enrollment, 1,859 (38.6%) of them had AF. Compared with the 2,953 patients without AF, AF patients were characterized by higher age (71 vs. 68 years), lower estimated glomerular filtration rate (58.9 vs. 61.9 ml/min/1.73 m2), higher brain natriuretic peptide (152 vs. 74.5 pg/ml), similar left ventricular ejection fraction (56.8 vs. 56.5%), and a similar prescription rate of β-blockers (48.1 vs. 50.6%) and renin-angiotensin system (RAS) inhibitors (72.9 vs. 71.6%). Among the patients without AF at enrollment, 106 (3.6%) developed new AF during the median 3.2-year follow-up, which was associated with increased mortality (adjusted hazard ratio, 1.72; P=0.013). In contrast, neither paroxysmal nor chronic AF at enrollment was associated with increased mortality. The mortality rate was significantly high in the first year after the onset of new AF. On inverse probability of treatment weighting analysis using propensity score, RAS inhibitors and statins were associated with reduced incidence of new AF, and diuretics were associated with increase of new AF.Conclusions:Onset of new AF, but not a history of AF, is associated with increased mortality in CHF patients, especially in the first year. (Circ J 2016; 80: 157–167)