In Japan, diabetes mellitus was called 'Shoukachi', thirst disease, before the 18th century. Western medicine was first introduced by the Portuguese in the 16th century. During the period of National Isolation from the 16th to the middle of the 19th century, Nagasaki served as the sole window of Japan, through which Dutch medicine came into Japan. After the Meiji Reform in the 19th century, German medicine was actively introduced. There remains a record of a lecture by Prof. Aoyama on diabetes mellitus delivered in 1906, in which the restriction of carbohydrate intake and a meat-rich diet was recommended. Prof. Sakaguchi, the successor of Prof. Aoyama and a pioneer of diabetes research in Japan, studied blood sugar profiles in healthy and diabetic subjects and invented a rice-meal test to examine glucose tolerance. In 1928, he wrote 'diabetes occurs by the deficiency of insulin, genetic factors are important, and the bodily need for insulin is increased with obesity'. He and his colleagues found that too low an intake of carbohydrate may impair glucose tolerance. After the 2nd World War, Japanese doctors eagerly learned foreign medicine, especially from the United States. Since the 1940s, a new concept has arisen in which diabetes was defined by blood glucose values after glucose loading. But, in 1970, a Committee of the Japan Diabetes Society (JDS) expressed an opinion that diabetes should not be solely defined by the results of the glucose tolerance test but should be considered as a disease or diseases with characteristic metabolic and clinical features, based on deficient insulin action.(ABSTRACT TRUNCATED AT 250 WORDS)