Determining the cardioprotective effects of aspirin (acetylsalicylic acid) remains a focus for both basic science and clinical investigation. Although other contributors are probably present, the favorable prostacyclin-to-thromboxane ratio induced by low-dose aspirin appears beneficial for reducing cardiovascular mortality associated with unstable angina and myocardial infarction. The precise dosage, frequency and timing of aspirin's administration to reduce the incidence of vaso-occlusive events remains to be determined. This article reviews aspirin's mechanism of action and use for the prevention of myocardial infarction.