Staphylococcus lugdunensis is a recently described coagulase-negative staphylococcus that has been associated with human infections, including nine reported cases of infective endocarditis. The present study describes 11 other cases of infective endocarditis caused by this organism. The infection occurred in patients whose mean age was 61 years and was community-acquired in most cases. A preexisting cardiac abnormality was present in eight patients. Three of the 11 infections involved prosthetic valves. Ten strains were susceptible to penicillin. The destructive course of the infection, the need for valve replacement, and the high mortality suggest that S. lugdunensis causes a virulent form of endocarditis.