We designed this study to explore to what extent the excess risk of cardiovascular events in diabetic individuals is attributable to hypertension. We retrospectively analyzed prospectively collected data from the Framingham original and offspring cohorts. Of the 1145 Framingham subjects newly diagnosed with diabetes mellitus who did not have a previous history of cardiovascular events, 663 (58%) had hypertension at the time that diabetes mellitus was diagnosed. During 4154 person-years of follow-up, 125 died, and 204 experienced a cardiovascular event. Framingham participants with hypertension at the time of diabetes mellitus diagnosis exhibited higher rates of all-cause mortality (32 versus 20 per 1000 person-years; P<0.001) and cardiovascular events (52 versus 31 per 1000 person-years; P<0.001) compared with normotensive subjects with diabetes mellitus. After adjustment for demographic and clinical covariates, hypertension was associated with a 72% increase in the risk of all-cause death and a 57% increase in the risk of any cardiovascular event in individuals with diabetes mellitus. The population-attributable risk from hypertension in individuals with diabetes mellitus was 30% for all-cause death and 25% for any cardiovascular event (increasing to 44% and 41%, respectively, if the 110 normotensive subjects who developed hypertension during follow-up were excluded from the analysis). In comparison, after adjustment for concurrent hypertension, the population-attributable risk from diabetes mellitus in Framingham subjects was 7% for all-cause mortality and 9% for any cardiovascular disease event. Although diabetes mellitus is associated with increased risks of death and cardiovascular events in Framingham subjects, much of this excess risk is attributable to coexistent hypertension.