Native valve endocarditis caused by coagulase negative staphylococci has become more common. A study of 35 cases showed that the infections were usually acquired in the community and occurred in men (mean age 51 years). A pre-existing cardiac abnormality (mitral leaflet prolapse in a third of patients) was detected in 26 (74%). The source of the organisms in the community acquired infections was assumed to be the skin, though lesions were seldom demonstrated; most hospital acquired infections resulted from intravenous devices. Community acquired organisms were usually sensitive to penicillin, whereas those acquired in hospital were often multiresistant. Most infections were caused by Staphylococcus epidermidis. The frequency of acute presentation (26%) and of major neurological abnormality (23%), together with the need for valve replacement (often emergency) (51%) and the mortality (36%) suggest that coagulase negative staphylococci can be virulent aggressive pathogens, mimicking Staphylococcus aureus.