We describe an outbreak of acute rheumatic fever that occurred in the intermountain area centered in Salt Lake City, Utah. Seventy-four children meeting the modified Jones criteria for the diagnosis of acute rheumatic fever were evaluated by the staff at Primary Children's Medical Center, Salt Lake City, from January 1985 through June 1986. This represents an eightfold increase over the average annual incidence at this hospital during the past decade. Carditis, a dominant feature of the outbreak, was confirmed by auscultation in 53 of the patients (72 percent). An additional 14 patients were found to have mitral regurgitation by Doppler ultrasound examination, raising the total incidence of carditis to 91 percent. The children were predominantly from white (96 percent) middle-class families with above-average incomes and with ready access to medical care. There was no apparent increase in the incidence of streptococcal disease or other explanation for the marked increase in acute rheumatic fever. However, mucoid M type 18 and M type 3 group A streptococcal strains were isolated from several siblings of the patients and from schoolchildren (chosen at random) in the area. We conclude that acute rheumatic fever remains an important health problem in the United States.