Previously, we have shown that a 5-mg/kg of body weight daily dose of aspirin (ASA) caused reductions in the bacterial densities and weights of aortic vegetations in a rabbit model of Staphylococcus aureus endocarditis. We sought to determine (i) whether ASA dosage influences the development of vegetations and (ii) whether ASA given with antimicrobial therapy improves the treatment outcome of infective endocarditis. To study the influence of ASA dosage, animals received either no ASA (control) or oral doses of 2.5, 10, 20, and 50 mg/kg daily. The 2.5- and 10-mg/kg groups had statistically significant reductions in vegetation weight compared with untreated controls. The 10-mg/kg dose also resulted in a significant decrease in bacterial densities compared with those of the controls. Although reductions in weight and bacterial density were observed in other ASA-treated groups, these did not achieve statistical significance. To study the influence of ASA and antimicrobial therapy, the animals received either vancomycin alone or vancomycin with ASA. When ASA was given prior to and during antimicrobial therapy, a significant reduction in vegetation weight was observed. Additionally, the rate of sterilization was directly proportional to this observed reduction in weight. ASA's impact on the reduction of both the bacterial density and the weight of aortic vegetations is a dose-dependent phenomenon. When given with antimicrobial therapy, ASA not only reduces vegetation weight but also improves the rate of sterilization. This study provides additional data regarding the role of ASA in the treatment of endocarditis.