Background: Atrial fibrillation (AF) is the most common supraventricular tachyarrhythmia in patients with acute myocardial infarction (AMI). However, little is known about the impact of AF on in-hospital and long-term mortalities in patients with AMI in the era of primary percutaneous coronary intervention (PCI). Methods: Six hundred ninety-four consecutive patients with AMI admitted within 48 h after symptom onset were analyzed. All patients successfully underwent primary PCI at the acute phase of AMI. Patients were divided into 2 groups according to the presence of AF at admission or during index hospitalization. We retrospectively evaluated the in-hospital and long-term all-cause mortalities between patients with and those without AF. Results: AF was detected in 38 patients (5.5%) at admission and in 51 patients (7.3%) during hospitalization. Patients with AF were older and had a higher heart rate, lower ejection fraction, higher prevalence of hypertension, worse renal function, higher peak level of creatine phosphokinase, and lower rate of final TIMI flow grade 3 than those without AF. Although patients with AF had a more complicated clinical course and higher in-hospital mortality (11.2% vs. 4.0%, P=0.009), there was no significant association between presenting AF and in-hospital death after adjustment for baseline confounders (odds ratio, 2.63; 95% confidence interval [CI], 0.91–5.47; P=0.076). During the follow-up period of 3.0±1.7 years, patients with AF had a higher all-cause mortality than those without AF (30.3% vs. 22.1%, P=0.004 by log-rank test). However, after adjustment for clinical characteristics, presenting AF was not an independent predictor of all-cause mortality (hazard ratio, 1.15; 95% CI, 0.67–1.88; P=0.588). Conclusions: AF is a common complication of AMI and associated with a more complicated clinical course. However, AF is not an independent predictor of both in-hospital and long-term mortalities in the PCI era.