We retrospectively reviewed 55 episodes that fulfilled criteria forStaphylococcus aureusendocarditis in 50 drug addicts. The most common presenting symptoms were fever (90%), chest pain (58%), and cough (43%). All patients had evidence of right-sided heart involvement, and a murmur of tricuspid insufficiency was noted in 42%. Evidence of left-sided heart involvement was present in only 5%. The most helpful laboratory aid in facilitating an early clinical diagnosis of endocarditis was the chest x-ray film. Roentgenographic evidence of septic pulmonary emboli was present in 67% of initial chest films and eventually in 87% of all cases. All but five patients completed at least four weeks of intravenous antibiotic therapy. No patients required cardiac surgery and there were no deaths. The apparent predilection ofS aureusfor the right side of the heart and infrequent left-sided involvement may explain why addicts with endocarditis have a favorable response to antibiotic therapy.