Eighteen drug addicts with left-sided valvular endocarditis requiring operation are reviewed. Gram-positive bacteria were the most common organisms cultured (61%), with Staphylococcus aureus present in 7 of 11 patients. Gram-negative bacteria, exclusively Pseudomonas aeruginosa, were cultured in the remaining 39%. Indications for operation included sepsis (61%), heart failure (78%), and systemic emboli (22%). Abscesses formed in 6 of 11 patients with gram-positive endocarditis, while only one abscess was present with gram-negative endocarditis. Normal valves were infected in 17 of 18 patients (94%). Early surgical mortality (less than 30 days) was 11%. There were major complications in 79% of these patients, including persistent sepsis (50%), valvular dehiscence, prosthetic endocarditis or perivalvular leakage (37%), and mycotic aneurysms (22%). These complications were directly related to a late mortality of 44%, yielding an overall mortality of 50% in the first nine months after operation. Contrary to previous reports of acceptable surgical survival for valvular endocarditis, these data suggest that endocarditis involving the aortic or mitral valve in a drug addict is a highly lethal disease due to the virulence of the organisms, the severity of the complications encountered, and the predisposition to continued addiction.