The results of these randomized, controlled trials were available to the members of U.S. Preventive Services Task Force at the time of their 1996 recommendation (4–5). At that time, the Task Force found insufficient evidence to recommend for or against routine aspirin prophylaxis for the primary prevention of myocardial infarction in asymptomatic persons (6). Two additional large primary prevention trials were published in 1998, and another was reported in January 2001 [7–9]. In light of the new evidence, the U.S. Preventive Services Task Force sought to reassess the value of aspirin for the primary prevention of cardiovascular events. The Task Force's assessment was performed in partnership with the Agency for Healthcare Research and Quality, Rockville, Maryland, and investigators from the RTI-UNC Evidence-based Practice Center, Research Triangle Park, North Carolina. For this review, we examined three key questions: 1) Does aspirin chemoprevention in patients without known cardiovascular disease reduce the risk for myocardial infarction, stroke, and death? 2) Does aspirin chemoprevention increase major gastrointestinal bleeding, hemorrhagic strokes, or both? 3) What is the balance of benefits and harms for aspirin therapy in patients with different levels of risk for coronary heart disease? The analytic framework of the review can be found in Appendix Figure 1.