The National Alzheimer's Coordinating Center was established in 1999 by the National Institute on Aging/NIH to facilitate collaborative research. Using data collected from NIA-funded Alzheimer's Disease Research Centers (ADRCs) across the United States, NACC has developed and maintains a large relational database of standardized clinical and neuropathological research data. In partnership with the Alzheimer's Disease Genetics Consortium (ADGC), the National Centralized Repository for Alzheimer's Disease and Related Disorders (NCRAD), and the NIA Genetics of Alzheimer's Disease Data Storage Site (NIAGADS), NACC provides a valuable resource for both exploratory and explanatory Alzheimer's disease research. NACC data are freely available to all researchers.