Antiplatelet agents are central to the treatment and prevention of cardiovascular disease. Although aspirin is the most widely used agent, randomized trials have assessed whether adding clopidogrel to aspirin ("dual-antiplatelet therapy") offers additional benefit with acceptable safety. Unfortunately, these trials have reached conflicting results, in part because of the heterogenous populations they studied. To clarify the role of dual-antiplatelet therapy for patients with vascular disease, a systematic review and meta-analysis of randomized controlled trials was performed. Medline and the Cochrane Collaboration and American College of Physicians Journal Club databases were searched for trials published from 1966 to August 2006 that compared aspirin and clopidogrel with antiplatelet monotherapy. Only trials that presented clinically relevant efficacy and safety outcomes were included. From each trial, demographic data and outcomes were recorded. Summary odds ratios and 95% confidence intervals (CIs) were calculated using a random-effects model. Eight trials comprising 91,744 patients were included. Mean follow-up ranged from 28 days to 18 months. Compared with aspirin alone, dual therapy with aspirin and clopidogrel reduced the odds ratio of the composite outcome of death, reinfarction, and stroke by 15% (95% CI 23% to 6%) in patients with acute coronary syndromes and by 34% (95% CI 44% to 22%) in patients who underwent percutaneous coronary intervention. Dual therapy also significantly reduced the odds of fatal and nonfatal reinfarction in these patient groups but did not significantly reduce the odds of all-cause mortality. Dual therapy was associated with significantly increased risk for major bleeding in studies >1 month in duration (odds ratio 1.80, 95% CI 1.40 to 2.30). In conclusion, combining aspirin and clopidogrel significantly reduces the odds of major cardiovascular events in patients with acute coronary syndromes or those who undergo percutaneous coronary intervention but at the expense of significant increases in the risk for bleeding.