This study evaluated the impact of serum creatinine levels on in-hospital mortality in 1,359 consecutive patients with acute myocardial infarction (from a Japanese prospective multicenter registry) who underwent successful primary percutaneous coronary intervention (PCI). Even in the patients who underwent successful primary PCI, the in-hospital mortality of patients with mild (1.2 </= creatinine < 2.0 mg/dl) and severe (creatinine >/=2.0 mg/dl) renal dysfunction was greater (17.1% and 34.5%, respectively) than that of patients without renal dysfunction (3.9%) (relative risk [RR] 1.72, 95% confidence interval [CI] 0.94 to 3.14, p = 0.080; and RR 4.26, 95% CI 1.48 to 12.27, p <0.0001, respectively).