Epidemiological studies on diabetes mellitus revealed that the number of patients with diabetes mellitus is gradually increasing in Japan along with development of car society and westernization of food intake. Since prevalence of diabetes mellitus increases with aging, proportion of individuals with diabetes mellitus aged over 60 has exceeded two-third of estimated total number of patients (7.40 million in 2002) in Japan where aging of society is rapidly progressing. Type 2 diabetes mellitus is common in diabetes mellitus in old age, and there are rarely elderly patients with type 1 diabetes mellitus. Prevalence of both diabetic microangiopathy and atherosclerotic vascular diseases is higher in the elderly with diabetes mellitus than in the middle-aged with diabetes mellitus. Furthermore, atherosclerotic vascular diseases (ischemic heart disease, cerebro-vascular disease and peripheral vascular disease) are more prevalent in the elderly with diabetes mellitus than in those without diabetes mellitus. Many studies demonstrated that functional declines, i.e. decreases in activities of daily living, physical activity and cognitive function, deteriorated quality of life in the elderly, and functional declines are more prominent in the elderly with diabetes mellitus than in those without diabetes mellitus. In order to clarify how the elderly patients with diabetes mellitus should be treated to maintain their quality of life, a nationwide randomized controlled intervention study using 1173 Japanese elderly patients with diabetes mellitus is now performing. In summary, number of elderly patients with diabetes mellitus is overwhelmingly increasing in Japan as well as in westernized countries. It is necessary for us to treat the elderly with diabetes mellitus to maintain their function and quality of life.