Diabetes mellitus is associated with several-fold increased risk of cardiovascular events, including death, yet whether treatment of hyperglycemia can reduce this risk remains controversial.Epidemiologic associations between hyperglycemia and cardiovascular risk have been confirmed and extended to levels below those diagnostic of diabetes. Long-term follow-up of treatment early in diabetes suggests benefits may lag 10 years or more after initiation of treatment. Intensive glycemic treatment of high-risk persons with current methods may increase short-term mortality.Further analyses of existing data to define high-risk and lower-risk subgroups of patients may allow revision of treatment guidelines. Future studies may begin early in diabetes, require long-term follow-up, and use improved methods of treatment.