Described here are three cases of acute native valve endocarditis due to the coagulase-negative pathogen Staphylococcus lugdunensis with serious complications. Two of the three patients died despite optimal antibiotic therapy and cardiovascular surgery. These cases demonstrate the aggressive nature of S. lugdunensis and emphasize the importance of identifying coagulase-negative staphylococci to the species level and not considering the isolation of S. lugdunensis from normally sterile body fluids as contamination. On the contrary, when this organism is found in patients with endocarditis, early surgery should be considered. The possibility that this organism could be misidentified as S. aureus because of "autocoagulation" and that commercial identification systems may misidentify it as S. haemolyticus, S. hominis or S. warneri should also be remembered.