The traditional cut-off for the cardiothoracic ratio (CTR) by chest X-ray was not originally proposed as a prognostic variable. We investigated an optimal CTR cut-off that could predict clinical outcomes in patients with acute myocardial infarction (AMI). A total of 3,083 AMI patients (65.2 ± 12.0 years, 2,091 males) who underwent successful percutaneous coronary intervention were divided into two groups by use of a CTR of 0.42 as determined by receiver-operating characteristic curve analysis (group I: CTR ≤ 0.42, group II: CTR > 0.42). We compared the incidences of in-hospital death and major adverse cardiac events (MACEs), including cardiac death, reinfarction, coronary artery bypass grafting, and target lesion revascularization, during 12 months between the groups. The patients in group II were older than those in group I and included more women. The patients in group II were more likely to have hypertension and multivessel disease and had a higher Killip class, higher troponin, higher N-terminal pro-brain natriuretic peptide, and lower ejection fraction than did those in group I. The in-hospital death rate was higher in group II (1.9 vs. 4.8%, p < 0.001). The incidences of cardiac death and composite of MACEs during 12 months of follow-up were significantly higher in group II than in group I (2.4 vs. 5.7%, p < 0.001, and 16.0 vs. 19.8%, p = 0.007, respectively). Multivariable logistic regression analysis revealed that CTR greater than 0.42 was an independent predictor of MACEs (relative risk: 1.361, 95% CI 1.014-1.827, p = 0.040). A CTR greater than 0.42, although within the traditional normal range, was associated with worse in-hospital and long-term clinical outcome in AMI patients.