AIMS: To investigate the occurrence and prognostic significance of atrial fibrillation/-flutter following acute myocardial infarction. METHODS AND RESULTS: The occurrence and prognostic significance of atrial fibrillation/-flutter were studied in 6676 consecutive patients with acute myocardial infarction screened in 27 centres in Denmark for inclusion into the TRAndolapril Cardiac Evaluation (TRACE) study. Information about occurrence of atrial fibrillation/-flutter during hospitalization was prospectively collected for the following three periods: day 1-2, day 3-4 and from day 5 until discharge. A total of 1395 patients (21%) suffered from atrial fibrillation/-flutter in one or more of the specified periods during hospitalization. Patients with atrial fibrillation/-flutter were significantly older, a significantly greater proportion were women, left ventricular systolic dysfunction was more extensive, thrombolytic therapy was received less frequently, and anterior Q wave myocardial infarction was experienced more frequently than patients without atrial fibrillation/-flutter. History of acute myocardial infarction and/or angina pectoris was similar in patients with and without atrial fibrillation/-flutter, whereas significantly more patients with atrial fibrillation/-flutter had a history of hypertension, congestive heart failure, diabetes mellitus, pulmonary disease and stroke. The unadjusted in-hospital mortality rate was significantly higher in patients with atrial fibrillation/-flutter in one or more of the specified periods during hospitalization (18%) than in patients without atrial fibrillation/-flutter (9%), P<0.001. After adjustment for baseline characteristics, the presence of atrial fibrillation/-flutter was still associated with increased in-hospital mortality; odds ratio=1.5 (95% Cl: 1.2-1.8), P<0.001. In patients surviving hospitalization, the unadjusted 5-year mortality rate was also significantly higher in patients suffering from atrial fibrillation/-flutter (56%) than in patients without atrial fibrillation/-flutter (34%), P<0.001. After adjustment for important prognostic baseline characteristics, the presence of atrial fibrillation/-flutter was still associated with an increased mortality, relative risk=1.3 (95% Cl: 1.2-1.4). Subgroup analysis revealed that sustained atrial fibrillation/-flutter during hospitalization was associated with the highest risk of dying, relative risk=1.4 (95% Cl: 1.2-1.7). CONCLUSION: Atrial fibrillation/-flutter often occurs after acute myocardial infarction and our analysis demonstrated that it was an independent predictor of an increased short and long-term mortality.