In secondary prevention among a wide range of patients who have survived a prior occlusive vascular event, as well as during acute myocardial infarction and acute occlusive stroke, aspirin produces statistically significant and clinically important reductions in the risk of subsequent myocardial infarction, stroke, and vascular death. In primary prevention, aspirin reduces risk of a first myocardial infarction, but the data on stroke and vascular deaths remain inconclusive. In addition, the average absolute risk of subjects randomized in the primary prevention trials was so low that it is not possible to get reliable estimates of the benefit-to-risk ratio in primary prevention in subjects at moderate risk. Until the results of ongoing trials are available, nobody would disagree that a nonfatal myocardial infarction or stroke is more likely to be disabling than a nonfatal bleed. Thus, in primary prevention at present, the appropriate and judicious use of aspirin by clinicians based on individual clinical judgments that weigh their absolute benefits against the absolute risks of the drug, will avoid premature morbidity and possibly, mortality.