A records-linkage system has been established for residents of Rochester, Minnesota and surrounding Olmsted County in the United States. This includes medical data sources both within and outside the Mayo Clinic (located in Rochester) and involves the cooperation of all medical facilities in the area. Rochester residents have access to high quality medical care and most serious health problems come to medical attention. Although the population is relatively small, there is a high level of case ascertainment and follow-up. Such a records-linkage system provides a rich data base for studies of neurologic disease in a well-defined population. In many instances, Rochester data provide the only estimates of morbidity for a large number of neurological and neuromuscular disorders in the United States.