Background: In persons with diabetes, chronic hyperglycemia (assessed by glyco sylated hemoglobin level) is related to the development of microvascular disease ; however, the relation of glycosylated hemoglobin to macrovascular disease is l ess clear. Purpose: To conduct a meta analysis of observational studies of the association between glycosylated hemoglobin and cardiovascular disease in diabet ic persons. Data Sources: Search of the MEDLINE database by using Medical Subjec t Heading search terms and keywords related to glycosylated hemoglobin, diabete s, and cardiovascular disease. Study Selection: Prospective cohort studies with data on glycosylated hemoglobin levels and incident cardiovascular disease. Data Extraction: Relative risk estimates were derived or abstracted from each cohort study that met the inclusion criteria. Data Synthesis: Adjusted relative risk e stimates for glycosylated hemoglobin (total glycosylated hemoglobin, hemoglobin A1, or hemoglobin A1c levels) and cardiovascular disease events (coronary heart disease and stroke) were pooled by using random effects models. Three studies i nvolved persons with type 1 diabetes (n=1688), and 10 studies involved persons w ith type 2 diabetes (n=7435). The pooled relative risk for cardiovascular diseas e was 1.18; this represented a 1 percentage point increase in glycosylated hemo globin level (95%CI, 1.10 to 1.26) in persons with type 2 diabetes. Results in persons with type 1 diabetes were similar but had a wider CI (pooled relative ri sk, 1.15 [CI,0.92 to 1.43]). Limitations: This review largely reflects the lim it ations of the literature. Important concerns were residual confounding, the poss ibility of publication bias, the small number of studies, and the heterogeneity of study results. Conclusions: Pending confirmation from large, ongoing clinical trials, this analysis shows that observational studies are consistent with limi ted clinical trial data and suggests that chronic hyperglycemia is associated wi th an increased risk for cardiovascular disease in persons with diabetes. Background: In persons with diabetes, chronic hyperglycemia (assessed by glyco sylated hemoglobin level) is related to the development of microvascular disease; however, the relation of glycosylated hemoglobin to l ess clear. Purpose: To conduct a meta analysis of observational studies of the association between glycosylated hemoglobin and cardiovascular disease in diabetic persons. Data Sources: Search of the MEDLINE database by using Medical Subjec t Heading search terms and keywords related to glycosylated hemoglobin, diabete s, and cardiovascular disease. Study Selection: Prospective cohort studies with data on glycosylated hemoglobin levels and incident cardiovascular disease. Data Extraction: Relative risk estimates were derived or abstracted from each cohort study that met the inclusion criteria. Data Synthesis: Adjusted relative risk e stimates for glycosylated hemoglobin (total glycosylated hemoglobin, hemoglobin A1 , or hemoglobin A1c levels) and car Three studies of nodule persons with type 1 diabetes (n = 1688), and 10 studies involved persons w ith type 2 diabetes (n = 7435). The pooled relative risk for cardiovascular diseas e was 1.18; this represented a percentage point increase in glycosylated hemo globin level (95% CI, 1.10 to 1.26) in persons with type 2 diabetes. Results in persons with type 1 diabetes were similar but had a wider CI (pooled relative ri sk, 1.15 [CI, 0.92 to 1.43]). Limitations: This review largely reflects the lim it ations of the literature. Important concerns were residual confounding, the poss ibility of publication bias, the small number of studies , and the heterogeneity of study results. Conclusions: Pending confirmation from large, ongoing clinical trials, this analysis shows that observational studies are consistent with limi ted clinical trial data and suggests that chronic hyperglycemia is associated with an increased risk for cardiovascular disease in persons with diabetes.