The rationale for prophylaxis was based largely on expert opinion and what seemed to be a rational and prudent attempt to prevent a life-threatening infection. On the basis of the ACC and AHA Task Force on Practice Guide- lines evidence-based grading system for ranking recommen- dations, the recommendations in the AHA documents pub- lished during the past 50 years would be Class IIb, LOE C. Accordingly, the basis for recommendations for IE prophy- laxis was not well established, and the quality of evidence was limited to a few case-control studies or was based on expert opinion, clinical experience, and descriptive studies that utilized surrogate measures of risk. Over the years, other international societies have published recommendations and guidelines for the prevention of IE.14,15 Recently, the British Society for Antimicrobial Chemother- apy issued new IE prophylaxis recommendations.15 This group now recommends prophylaxis before dental procedures only for patients who have a history of previous IE or who have had cardiac valve replacement or surgically constructed pulmonary shunts or conduits. The fundamental underlying principles that drove the formulation of the AHA guidelines and the 9 previous AHA documents were that (1) IE is an uncommon but life- threatening disease, and prevention is preferable to treatment of established infection; (2) certain underlying cardiac con- ditions predispose to IE; (3) bacteremia with organisms known to cause IE occurs commonly in association with invasive dental, GI, or GU tract procedures; (4) antimicrobial prophylaxis was proven to be effective for prevention of experimental IE in animals; and (5) antimicrobial prophylaxis was thought to be effective in humans for prevention of IE associated with dental, GI, or GU tract procedures. The Committee believes that of these 5 underlying principles, the first 4 are valid and have not changed during the past 30 years. 