Diabetes mellitus is an established risk factor for coronary artery disease and ischemic stroke, and can induce micro- and macroangiopathy. Stroke is strongly associated with severe disability and mortality in Japan, and the risk of stroke is 2-4 times higher among patients with diabetes than among matched controls. The abnormal metabolic state in diabetes can cause arterial dysfunction, and patients with diabetes require treatment for each metabolic abnormality to attenuate atherogenesis. Moreover, diabetes affects stroke outcome as well. This review on diabetes mellitus and ischemic stroke discusses three main topics: epidemiology, pathophysiology, and treatment.