In 27 cases of endocarditis due to coagulase-positive Staphylococcus aureus, the mortality rate was 74%. The diversity of presenting symptoms and physical findings often gave little clue to the underlying illness. Failure to suspect the diagnosis early in the course of the disease may well have contributed to the high rate of mortality. The choice of the antibiotic which is most likely to prove effective is difficult to make early in the course of the disease, in view of the variable resistance of the staphylococci to antibiotics. It is recommended that a combination of antibiotics be given at the onset. The frequent repetition of blood cultures and sensitivity studies during the course of illness is of prime importance.