Strict criteria were used to identify all definite, probable, and possible cases of endocarditis in residents of Olmsted County, Minnesota, from 1950 through 1981. The mean annual age- and sex-adjusted incidence rates per 100,000 person-years were 3.8 for total cases and 3.2 for definite and probable cases only. Total rates were 4.3 for 1950 through 1959, 3.3 for 1960 through 1969, and 3.9 for 1970 through 1981. Rheumatic heart disease was the underlying disorder in 26% of cases, with a shift noted during 1970 through 1981 to involvement of prosthetic rather than natural valves in these patients. Mitral valve prolapse was identified in 17% of cases. No source of infection could be identified in 41% of cases, including half of those cases with rheumatic or congenital heart disease. In cases diagnosed prior to autopsy, the 60-day fatality fell from 46% during 1950 through 1959 to 22% and 26% during 1960 through 1969 and 1970 through 1981, respectively.