The Rochester Epidemiology Project is a unique medical records-linkage system that encompasses the care delivered to residents of Rochester and Olmsted County, Minnesota. It is the creation of Dr. Leonard T. Kurland, who envisioned the population-based data resource that would result from combining the clinical documentation developed by the Mayo Clinic with that obtained by other community providers, most notably the Olmsted Medical Group and its affiliated Olmsted Community Hospital. Kurland built on the Mayo unit medical record system that was designed by Dr. Henry S. Plummer in 1907 and on the medical and surgical indexing systems introduced by Dr. Joseph Berkson in 1935. By affording access to details of the medical care given to local residents, the Rochester Epidemiology Project is able to provide accurate incidence data for almost any serious condition and to support population-based analytic studies of disease causes and outcomes. Thus, epidemiologic studies of a wide array of disorders have been possible and have culminated in almost 900 publications since the system was organized in 1966. Olmsted County is one of the few places in the world where the occurrence and natural history of diseases can be accurately described and analyzed in a defined population for a half century or more.