The ability of aspirin to reduce cardiovascular disease risks has been tested in randomized trials in a wide range of patient categories. There are clear benefits of aspirin on nonfatal myocardial infarction, nonfatal stroke, and vascular death among patients with prior manifestations of cardiovascular disease, such as myocardial infarction, unstable angina, and stroke. Aspirin is also beneficial to those in the acute phase of evolving myocardial infarction. In primary prevention, there is a clear reduction in myocardial infarction in men. A large-scale primary prevention trial in women is presently underway.