46 Staphylococcus aureus endocarditis episodes diagnosed with strict criteria in non drug addict patients, and 25 episodes in drug addict patients have been comparatively analyzed. Infection was found in the left side of the heart in 87% of the non addict patients and in 16% of the addicts. On the contrary, 84% of the addicts had endocarditis of the tricuspid and pulmonary valves while only 13% of the non addicts had right heart involvement. The right side endocarditis in the non addicts was always due to intracardiac catheters. 54% of the endocarditis episodes in the non addicts were fatal. Only two addicts, both when had left side endocarditis, died. Mortality was conditioned by infection of the left side of the heart as well as by the existence of heart failure. No significant differences were found between the evolution of patients treated with only one agent or of those treated with a beta-lactam antibiotic plus gentamicin. The emergency valve replacement significantly improved the prognosis of patients with prosthetic valve endocarditis.