Bacterial endocarditis is a preventable disease if physicians and susceptible patients cooperate in exercising diligent prophylaxis. Since prevention is easier and more certain than cure, it is incumbent on physicians to denote persons at risk and to institute preventive chemotherapy under appropriate circumstances. Structural abnormalities of the heart or great vessels create susceptibility; manipulations that free bacteria into the bloodstream increase the risk of the disease. Most commonly, bacterial endocarditis follows dental procedures or genitourinary and gastrointestinal tract surgery or instrumentation. The most frequent infecting organism is the a-hemolytic Streptococcus , and therefore, the most effective prophylaxis is penicillin. For a small minority of those afflicted with bacterial endocarditis, penicillin may not be the most effective drug, and these patients offer a special challenge to the physician's therapeutic skill. The importance of preventing bacterial endocarditis is such that the American Heart Association has revised and updated its statement on this subject.