Background: Recent studies have proven that initial renal dysfunction and worsening renal function during hospitalization can predict the clinical outcome of patients with acute myocardial infarction (AMI). There is limited study regarding acute kidney injury (AKI) by the RIFLE classification (Risk of renal failure, Injury to the kidney, Failure of kidney function, Loss of kidney function, and End-stage renal failure) to assess the outcome of AMI survivors. Methods and Results: During a mean follow-up period of 635.3±204.9 days, the 2-year mortality rate was 10.6% in 613 AMI patients who survived to discharge. Adjusted Cox regression analysis revealed that left ventricular dysfunction (<40%) [hazard ratio (HR), 2.83; 95% confidence interval (CI), 1.11–7.20; P=0.029], estimated glomerular filtration rate <60ml·min^–1·1.73m⁻² on admission (HR, 4.01; 95% CI, 1.51–10.62; P=0.005), and AKI severity by RIFLE classification during first week after hospitalization (Injury: HR, 8.11; 95% CI, 2.53–26.05; P=0.001; Failure: HR, 19.28; 95% CI, 2.24–166.26; P=0.007) were independent predictors of 2-year mortality. Conclusions: Independent of initial renal dysfunction on admission, the AKI severity by RIFLE classification may be useful in establishing the hospital discharge risk score for predicting long-term mortality in AMI patients who survive to discharge.