Background: Atrial fibrillation (AF) in heart failure (HF) is generally considered a negative prognostic factor. Recent studies indicate that the incidence of AF might be decreased by renin angiotensin aldosterone system inhibitors. The identification of a treatment to prevent its occurrence is likely to improve patients outcome. The aims of these subanalyses of Val-HeFT were to assess (a) the effects of valsartan in the prevention of AF, (b) the independent predictors of this event, and (c) the prognostic role of AF occurrence. Methods and Results: The occurrence of AF was evaluated based on adverse event reports in the patients with HF enrolled in Val-HeFT. Patients were randomized to valsartan or placebo on top of their prescribed treatments for HF. During the mean 23 months of follow-up, AF was reported in 287/4395 patients (6.53%) in sinus rhythm at baseline, of whom 113/2205 (5.12%) were allocated to valsartan and 174/2190 (7.95%) to placebo (P =. 0002). Multivariable analysis showed that brain natriuretic peptide (BNP) levels at baseline above the median value (HR 2.28, 95% CI 1.75-2.98), age over 70 years (HR 1.51, 95% CI 1.17-1.95), male sex (HR 1.53, 95% CI 1.07-2.18), and the valsartan treatment (HR 0.63, 95% CI 0.49-0.81) were independently associated with AF occurrence. Cox multivariable regression analysis showed that occurrence of AF was independently associated with a worse prognosis, with the adjusted hazard risks for all-cause mortality and combined mortality/morbidity of 1.40 (95% CI 1.16-1.58) and 1.38 (95% CI 1.12-1.70), respectively. Conclusions: The results of the present study demonstrate that (a) adding valsartan to prescribed therapy for HF significantly reduces the incidence of AF by 37%; (b) BNP level and advanced age were the strongest independent predictors for AF occurrence; and (c) AF occurrence further worsens the outcome in patients with HF.