Chronic kidney disease (CKD) is highly prevalent worldwide and is associated with an increased risk for adverse outcomes in patients hospitalized with acute coronary syndrome (ACS). In studies including thousands of patients admitted with myocardial infarction, CKD consistently determines a poorer prognosis for ACS patients. In contrast with CKD, information about the effect of acute kidney injury (AKI) on clinical outcomes after ACS is limited. Most data come from retrospective registry databank studies of nonconsecutive patients with a significant number of patients excluded from analyses. There are no prospective studies designed to determine whether AKI strictly diagnosed by the new the Acute Kidney Injury Network (AKIN) or RIFLE (Risk, Injury, Failure, Loss, and End-stage kidney disease) criteria is a risk factor for death after ACS, and there are no data comparing the RIFLE and AKIN criteria for AKI diagnosis after myocardial infarction. This article reviews the most important data on CKD and ACS and the available data on AKI and ACS. The importance of obtaining an early serum creatinine level after admission for ACS and the importance of renal function monitoring during hospitalization are stressed.