Hospital-acquired acute renal failure (ARF) is associated with an unacceptably high mortality rate. Depending on the severity of ARF (dialysis- or non-dialysis-requiring) and underlying disease, short-term mortality rates range from 30-70% or more, and have remained at these levels for several decades. While ARF may develop in 5% or more of hospitalized patients, the heterogeneity of ARF and accompanying comorbidity and severity of acute illness make it difficult to study. Clinical trials in ARF are needed to guide and improve practice, but fundamental information such as the appropriate definition of ARF is lacking. In this study, we propose to derive evidence-based definitions of hospital-acquired ARF and determine the independent associations among ARF, mortality, complications, length of stay and costs. We will determine whether there are important interactions among underlying disease processes and the estimated effects of ARF on mortality, length of stay and costs using alternative ARF definitions. We will determine predictors of multiple evidence-based definitions of ARF using information available in computerized medical records. We will determine the incidence and estimated effects of community-acquired ARF using alternative ARF definitions. Finally, we will evaluate the cost-effectiveness of theoretical agents for the prevention and treatment of ARF under alternative definitions. Data sources include an existing dataset based on hospitalized patients at Brigham and Women's Hospital (BWH), and data from a planned 12-month prospective study collecting administrative data on hospitalized and ambulatory patients at BWH and UCSF Medical Center. Medical record review of a random sample of ARF cases and controls will be incorporated to determine the adequacy of proxies for comorbidity and etiology of ARF. This study will efficiently inform the Nephrology community by providing a foundation of fundamental information (ARF definitions, incidence, consequences and costs) upon which a clinical trials network can be built.