BACKGROUND: Reports on the prognostic importance of atrial fibrillation following myocardial infarction have provided considerable variation in results. Thus, this study examined the impact of left ventricular systolic function and congestive heart failure on the prognostic importance of atrial fibrillation in acute myocardial infarction patients that might explain previous discrepancies. METHODS: The study population was 6676 patients consecutively admitted to hospital with acute myocardial infarction. Information on the presence of atrial fibrillation/flutter, left ventricular systolic function and congestive heart failure were prospectively collected. Mortality was followed for 5 years. RESULTS: In patients with left ventricular ejection fraction<0.25, atrial fibrillation/atrial flutter was associated with an increased in-hospital mortality (OR=1.8 (1.1-3.2); p<0.05) but not an increased 30-day mortality. In patients with 0.250.35. In patients with congestive heart failure, atrial fibrillation/atrial flutter was associated with an increased in-hospital mortality (OR=1.5 (1.2-1.9); p<0.001) and increased 30-day mortality (OR=1.4 (1.1-1.7); p<0.001) but not in patients without congestive heart failure. In hospital survivors, atrial fibrillation/atrial flutter was associated with an increased long-term mortality in all subgroups except those with left ventricular ejection fraction<0.25. CONCLUSIONS: Atrial fibrillation/atrial flutter is primarily associated with increased in-hospital mortality in heart failure patients. Long-term mortality is increased in all subgroups except those with left ventricular ejection fraction<25%.