Until recently, 5 major studies have formed the basis for the use of aspirin (acetylsalicylic acid) in primary prevention of cardiovascular (CV) events. Despite these data, the role of aspirin in primary prevention has not been established firmly. Six randomized trials have evaluated the benefits of aspirin for the primary prevention of CV events: the British Doctors' Trial, the Physicians' Health Study, the Thrombosis Prevention Trial, the Hypertension Optimal Treatment study, the Primary Prevention Project, and the Women's Health Study. The combined sample consists of 47,293 subjects on aspirin and 45,580 not on aspirin or placebo. A meta-analysis of these 6 trials assessed 6 CV end points: total coronary heart disease (CHD), nonfatal myocardial infarction (MI), total CV events, stroke, CV mortality, and all-cause mortality. No covariate adjustment was performed and appropriate tests for treatment effect, heterogeneity, and study size bias were applied. Using odds ratios and confidence intervals, the meta-analysis suggested superiority of aspirin for total CHD, nonfatal MI, and total CV events (p < or =0.001 in each case), with a nonsignificant trend (0.07 < p <0.34) for decreased risk of stroke, CV mortality, and all-cause mortality. There was no evidence of statistical bias (p >0.05). Given the study size and cohort, aspirin decreased the risk of CV events in this large patient sample. In conclusion, primary prevention with aspirin decreased the risk of total CHD, nonfatal MI, and total CV events, but there were no significant differences in the incidences of stroke or CV mortality.