Acute kidney injury is a common complication of acute myocardial infarction and is generally associated with adverse outcomes. We studied the incidence and clinical significance of transient versus persistent acute kidney injury in 1957 patients who survived an ST-elevation acute myocardial infarction. We divided the patients into 5 groups based on changes in serum creatinine level during hospitalization. Mild acute kidney injury (creatinine 0.3-0.49 mg/dl above baseline) occurred in 156 patients and was transient (resolved during their hospital stay) in 61. Moderate/severe acute kidney injury (creatinine more than or 0.5 mg/dl above baseline) was found in 138 patients and was transient in 60. Compared to patients without acute kidney injury, the adjusted hazard ratio for mortality was 1.2 in patients with mild, transient acute kidney injury and 1.8 in patients with mild, persistent injury where the creatinine remained elevated. Patients with persistent moderate/severe acute kidney injury had the highest mortality (hazard ratio 2.4), whereas patients with transient moderate/severe injury had an intermediate risk (hazard ratio of 1.7). A similar relationship was present between acute kidney injury and admissions for heart failure. Our study shows that dynamic changes in renal function during acute myocardial infarction are strongly related to long-term mortality and heart failure.