OBJECTIVE- The negative results of two randomized controlled trials (RCTs) have challenged current guideline recommendations for using aspirin for primary prevention of cardiovascular events among patients with diabetes. We therefore sought to determine if the effect of aspirin for primary prevention of cardiovascular events and mortality differs between patients with and without diabetes. RESEARCH DESIGN AND METHODS- We conducted a systematic search of MEDLINE, EMBASE, Cochrane Library, Web of Science, and Scopus since their inceptions until. November 2008 for RCTs of aspirin for primary prevention of cardiovascular events. Blinded pairs of reviewers evaluated studies and extracted data. Random-effects meta-analysis and Bayesian logistic regression were used to estimate the ratios of relative risks (RRs) of outcomes of interest among patients with and without diabetes. A 95% CI that crosses 1.00 indicates that the effect of aspirin does not differ between patients with and without diabetes. RESULTS- Nine RCTs with moderate to high methodological quality contributed data to the analyses. The ratios of RRs comparing the benefit of aspirin among patients with diabetes compared with patients without diabetes for mortality, myocardial infarction, and ischemic stroke were 1.12 (95% CI 0.92-1.35), 1.19 (0.82-1.17), and 0.70 (0.25-1.97), respectively. CONCLUSIONS- Whereas estimates of benefit among patients with diabetes remain imprecise, our analysis suggests that the relative benefit of aspirin is similar in patients with and without diabetes. © 2009 by the American Diabetes Association.