Infective endocarditis is a serious disease and should be, if possible, prevented. Two risk groups are classified in relation to the patient's underlying cardiac lesions. At high risk are patients with prosthetic valves or with a previous infective endocarditis. Patients with congenital and acquired heart disease, mitral valve prolapse with regurgitation and hypertrophic obstructive cardiomyopathy are at moderate risk. Patients of these two groups should receive antibiotic prophylaxis before dental or surgical procedures that cause bacteremia. For patients at moderate risk a single dose of an orally administered antibiotic should be given one hour before the procedure (e.g. amoxicillin 3 g for procedures of the oropharyngeal, gastrointestinal or genitourinary tract, where the causitive agents of endocarditis are Viridans streptococci or enterococci). Multiple doses are recommended for patients at high risk. The combination of amoxicillin and gentamicin (vancomycin and gentamicin in penicillin-allergic patients) offers the widest margin of safety in high-risk patients.