The incidence of acute rheumatic fever (ARF) in the United States has been steadily declining for three decades, and by the mid-1980s had virtually disappeared. In the past 3 years, however, several scattered outbreaks have occurred among US schoolchildren, and there is speculation that ARF will again become a major public health problem in North America. We report a clustering of 10 cases of ARF in young men undergoing basic training at a recruit camp in San Diego, Calif. This was the first such outbreak in two decades among US military personnel and the largest reported series of ARF cases among adults in more than a decade in the United States. Adult-onset ARF was not benign in our patients. The rate of carditis (3 of 10 patients) was similar to that in older studies of adults with ARF. All patients in our series suffered severe polyarthritis, anemia, and fever, but only 60% gave any history of preceding pharyngitis. Rheumatic fever must again be suspected in adults, as well as children, who present with polyarthritis.