Background Arrhythmias are common in chronic heart failure and affect outcomes. The incidence and significance of new arrhythmias in acute heart failure, however, are largely unknown. Methods and results The Outcomes of a Prospective Trial of Intravenous Milrinone for Exacerbations study randomized 949 patients with decompensated heart failure to receive intravenous milrinone or placebo. In the study, patients were divided into 2 groups based on the occurrence of a new arrhythmic event during their index hospitalization and analyzed for outcome. There were 59 new arrhythmic events occurring in 6% of the population. Of these, 49% were atrial fibrillation/flutter. The primary endpoint of days hospitalized for cardiovascular causes within 60 days after randomization was 30.9±22.7 for those in the arrhythmia group and 11.3±12.7 days for those with no arrhythmias (P=.0001). Mortality during index hospitalization was 26% in the arrhythmia group and 1.8% in the no arrhythmia group (P=.001). Death or hospitalization at 60 days was also worse in the arrhythmia group (35 versus 8.2%, P=.0001; 57 versus 34%, P=.001, respectively). Cox proportional hazard analysis identified new arrhythmias as an independent risk factor for the primary endpoint and death at 60 days. Conclusion New arrhythmia during an exacerbation of heart failure identifies a high-risk group with higher intrahospital and 60-day morbidity and mortality.