The effects of low-dose aspirin on cardiovascular disease have been tested in randomized trials in 3 types of populations: (1) patients with a history of cardiovascular disease; (2) patients with evolving acute myocardial infarction (MI), and (3) apparently healthy subjects. In a very wide range of patients with prior occlusive cardiovascular disease, aspirin reduces the risks of nonfatal MI, nonfatal stroke, and vascular death. Initiating aspirin therapy within 24 hours after the onset of symptoms of an acute MI also confers conclusive reductions in the risk of nonfatal reinfarction, nonfatal stroke, and total cardiovascular death. In primary prevention trials, aspirin has been shown to reduce the risk of a first MI in men, but the data on stroke and total cardiovascular death are not sufficient to allow firm conclusions to be drawn; randomized data from studies in women are not yet available. The Women's Health Study, an ongoing large-scale trial in female health care professionals, will provide the data necessary to assess the balance of benefits and risks of aspirin in primary prevention. Until then, the decision to use aspirin in primary prevention should be based on the clinical judgment of the physician and considered as an adjunct in the management of other cardiovascular disease risk factors.