Various definitions of acute kidney injury (AKI) exist, making comparisons among studies difficult. Despite this, significant changes have occurred in the epidemiology of AKI during the last decade. Recent studies, including PICARD and BEST, have examined the epidemiology of ICU-related AKI in the USA and worldwide, respectively, and found that AKI remains a major cause of morbidity and mortality. The incidence of AKI has increased, most likely due to a trend toward older, more severely and chronically ill patients admitted to the hospital. Sepsis and multi-organ system failure continue to be strongly associated with AKI, as well as pre-morbid chronic kidney disease. The proportion of patients with AKI requiring dialysis is high. The mortality of ICU-related AKI, although still very elevated, may be decreasing. Understanding these changes, in the context of standardized definitions, will be essential for the design of successful interventional studies. Copyright © 2008 S. Karger AG.