This study was carried out within the boundaries of Olmsted County, MN. Epidemiologic research in the county is possible because it is relatively isolated from other urban centers, and nearly all medical care is delivered to local residents by few providers. With the exception of a higher proportion of the working population employed in the health-care industry, the characteristics of the population of Olmsted County are similar to those of US whites. The Mayo Clinic provides approximately half of the primary care and nearly all specialty care for the community. Olmsted Medical Center and its affiliated hospital, along with the Mayo Clinic, provide comprehensive care for the region in every discipline. The epidemiologic potential in the community is enhanced by the fact that each provider uses a comprehensive medical record system, whereby all data collected on an individual are assembled in one place. Thus, the details of every inpatient and outpatient encounter can be accessed. Data are easily retrievable because the Mayo Clinic has maintained since the early 1900s extensive indexes based on clinical and histologic diagnoses and surgical procedures. Since 1966, similar indexes have been developed for non-Mayo providers under the aegis of the Rochester Epidemiology Project. This linkage system thus constitutes a unique opportunity to ensure complete ascertainment of tests in defined disease processes.