During a prospective 18-month study we saw 23 cases of bacterial endocarditis in heroin addicts. The infecting organisms were Staphylococcus aureus (16), Streptococcus faecalis (3), Strep. viridans (2), Pseudomonas (1), and mixed streptococci and Enterobacter (1). Solitary tricuspid involvement with Staph. aureus was seen in 11 patients; all had septic pulmonary emboli, but only 5 had splenomegaly, 5 had hematuria, and 1 had peripheral stigmata in the conjunctivas or nail beds. Of seven patients with left-sided endocarditis (mitral or aortic valve disease), all had hematuria, five had peripheral stigmata, and four had splenomegaly. Complications of endocarditis included aortic valve rupture, empyema, severe respiratory insufficiency, embolus to central nervous system with hemiparesis, gastrointestinal bleeding, septic joints, and renal failure. All patients survived, including three in whom the initial antibiotic failed and three who subsequently relapsed and required retreatment. Improved survival can be obtained with early diagnosis and intensive management of complications.