Background:Atrial fibrillation (AF) is considered to be associated with poor clinical outcomes in patients with acute myocardial infarction (AMI). However, it remains uncertain whether transient new-onset AF (NOAF) during AMI has a subsequent increased risk of poor clinical outcomes.Methods and Results:Transient NOAF was defined as AF that developed during AMI without a prior history and not documented for 1 month after discharge. The primary endpoints were major adverse cardiac events (MACE) and all-cause death. We enrolled 2,105 consecutive AMI patients. Overall, AF was observed in 209 (9.9%) and transient NOAF occurred in 102 (4.8%) among 150 patients (7.1%) with NOAF. The transient NOAF group showed higher 1-month (21.8 vs. 7.0%, P<0.001), 2-year (37.8 vs. 20.7%, P<0.001), and 5-year MACE rates (51.8 vs. 28.0%, P<0.001) than the group without AF. In-hospital (16.7 vs. 5.2%, P<0.001), 1-month (17.9 vs. 5.7%, P<0.001), 2-year (30.0 vs. 11.6%, P<0.001), and 5-year mortality rates (36.9 vs. 14.0%, P<0.001) were also higher in patients with transient NOAF. Transient NOAF was a significant independent predictor of both MACE (hazard ratio [HR] 1.55, 95% confidence interval [CI] 1.10–2.18, P=0.013) and death (HR 1.87, 95% CI 1.22–2.85, P=0.004).Conclusions:Transient NOAF was associated with the poorer clinical outcomes and was an important independent predictor of MACE and death in AMI patients. (Circ J 2016; 80: 1615–1623)