Aspirin has been tested for its benefit in preventing cardiovascular disease in randomized trials in three categories of patients. In secondary prevention among those with a history of myocardial infarction (MI), stroke or transient cerebral ischemia, or unstable angina pectoris, 25 randomized trials demonstrated significant reductions from aspirin of 25% for the occurrence of an "important vascular event" (nonfatal MI, nonfatal stroke, or vascular death), 32% for nonfatal MI, 27% for nonfatal stroke, and 15% for vascular mortality. Among those evolving an MI, the Second International Study of Infarct Survival (ISIS-2) showed a significant reduction of 23% in five-week vascular mortality among those started on a one-month regimen of daily aspirin within 24 hours of the onset of symptoms of suspected MI. Aspirin also significantly reduced reinfarction, nonfatal stroke, and important vascular events. Finally, in primary prevention, the US Physicians' Health Study (PHS) showed a significant 44% reduction in the occurrence of a first MI among apparently healthy male physicians; numbers of strokes and vascular deaths were insufficient to permit conclusions for these endpoints. Thus, aspirin is of clear benefit in reducing MI, stroke, and vascular death in secondary prevention and among those evolving an MI. It is also beneficial in the primary prevention of MI among men over 40, but data concerning its effects on stroke and vascular death remain inconclusive.