A follow-up study of 1939 diabetic patients with a mean observation period of 9.4 years was carried out in Osaka, Japan. The mortality rates per 1000 person-years were 31.35 for males and 21.99 for females, and the ratios of observed to expected number of deaths were 1.69 for males and 1.74 for females, indicating an excess mortality for diabetic patients of both sexes and higher mortality in males than in females in Japan. Factors related to the prognosis of the patients were age, elevated fasting glucose level, lower obesity index, hypertension, diabetic retinopathy, and albuminuria at entry to the study. Insulin treatment was also associated with poor prognosis. Cerebro-cardiovascular and renal disease were the major causes of death in diabetic patients; heart disease killed 19.5%, cerebrovascular disease 16.7% and renal disease 13.1%. The relatively high frequency of renal disease as a cause of death in type 2 diabetes, especially in patients with a lower age of onset, was noteworthy, suggesting some difference in the clinical manifestations of diabetes between Japan and Western countries. Malignant neoplasms accounted for 25% of deaths, and cirrhosis of the liver for 6.4%.