Classification of Recommendations: Class I: Conditions for which there is evidence and/or general agreement that a given procedure or treatment is beneficial, useful, and effective. Class II: Conditions for which there is conflicting evi- dence and/or a divergence of opinion about the usefulness/ efficacy of a procedure or treatment. Class IIa: Weight of evidence/opinion is in favor of usefulness/efficacy. Class IIb: Usefulness/efficacy is less well established by evidence/opinion. Class III: Conditions for which there is evidence and/or general agreement that a procedure/treatment is not useful/ effective and in some cases may be harmful. Level of Evidence: Level of Evidence A: Data derived from multiple random- ized clinical trials or meta-analyses. Level of Evidence B: Data derived from a single random- ized trial or nonrandomized studies. Level of Evidence C: Only consensus opinion of experts, case studies, or standard of care. History of AHA Statements on Prevention of IE The AHA has made recommendations for the prevention of IE for more than 50 years. In 1955, the first AHA document on this subject was published in Circulation.6 Table 1 shows a summary of the documents published from 1955 to 1997.1,6 13 The 1960 document called atten- tion to the possible emergence of penicillin-resistant oral microflora as a result of prolonged therapy for prevention of IE, and pediatric patients were included for the first time.8 Chloramphenicol was recommended for patients who were allergic to penicillin. 