Okell and Elliott23 in 1935 reported that 11% of patients with poor oral hygiene had positive blood cultures with viridans group streptococci and that 61% of patients had viridans group streptococcal bacteremia with dental extraction. As a result of these early studies and subsequent studies, during the past 50 years, the AHA guidelines recommended antimicrobial prophylaxis to prevent IE in patients with underlying cardiac conditions who underwent bacteremia- producing procedures on the basis of the following factors: (1) bacteremia causes endocarditis; (2) viridans group strep- tococci are part of the normal oral flora, and enterococci are part of the normal GI and GU tract flora; (3) these microor- ganisms were usually susceptible to antibiotics recommended for prophylaxis; (4) antibiotic prophylaxis prevents viridans group streptococcal or enterococcal experimental endocardi- tis in animals; (5) a large number of poorly documented case reports implicated a dental procedure as a cause of IE; (6) in some cases, there was a temporal relationship between a dental procedure and the onset of symptoms of IE; (7) an awareness of bacteremia caused by viridans group strepto- cocci associated with a dental procedure exists; (8) the risk of significant adverse reactions to an antibiotic is low in an individual patient; and (9) morbidity and mortality from IE are high. Most of these factors remain valid, but collectively, they do not compensate for the lack of published data that demonstrate a benefit from prophylaxis. Bacteremia-Producing Dental Procedures The large majority of published studies have focused on dental procedures as a cause of IE and the use of prophylactic antibiotics to prevent IE in patients at risk. Few data exist on the risk of or prevention of IE associated with a GI or GU tract procedure. 