We retrospectively reviewed 55 episodes that fulfilled criteria for Staphylococcus aureus endocarditis 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 of S aureus for the right side of the heart and infrequent left-sided involvement may explain why addicts with endocarditis have a favorable response to antibiotic therapy.