While acute kidney injury (AKI), a newer term for acute renal failure, has long been recognized as a common and serious complication of hospitalized patients, the study of AKI epidemiology has lagged. An important advance took place with the introduction of consensus AKI definitions by expert panels—first the Risk, Injury, Failure, Loss, and ESRD (RIFLE) criteria by the Acute Dialysis Quality Initiative in 2004,1 and then the Acute Kidney Injury Network (AKIN) criteria in 2007.2 These have allowed researchers to examine AKI epidemiology using a common case definition and to overcome one important limitation in the prior literature when cases were defined using different criteria in different studies, rendering it difficult to interpret variations in reported disease incidence. Since then, several studies have characterized AKI incidence in various settings, with wide range of incidences across different hospital and intensive care unit cohorts,3,4 but relatively few studies have examined secular trends in AKI epidemiology.