There has been a marked decline in mortality due to rheumatic fever in the United States. We present evidence for the important role of penicillin in changing the severity of rheumatic carditis, beginning about 1946. Since that year, mortality due to rheumatic carditis has rapidly decreased to zero at the hospital we studied (House of the Good Samaritan, Boston), the rate of loss of all murmurs in patients at the study hospital accelerated simultaneously and exceeded 40 percent by 1970, and the rates of decline in national mortality due to rheumatic carditis accelerated fourfold with the advent of antibiotics. These data, together with reports of recent outbreaks of rheumatic fever, emphasize the importance of continued efforts to diagnose and treat Group A streptococcal pharyngitis.