The projected numbers of patients with diabetes mellitus (ICD 9th; 250) 15 years from now were estimated. First, the numbers of patients with the disease in 1984, 1987, 1990, and 1993 were calculated by age and sex using data from the National Patient Surveys conducted by the Ministry of Health and Welfare. Then, population prevalence for calendar years 1996, 1999, 2002, 2005, and 2008 were estimated based on the past data using linear regression models. Finally, the total numbers of patients were calculated from the estimated prevalence multiplied by the estimated population figure of the national government. The prevalence and the numbers of patients are estimated to increase, and the numbers will be 1.7 million among males and 1.5 million among females in 2008. Besides, because of the increases of both the aged population and the disease prevalence, the proportion of patients aged 65 years or over will become as large as 40% of total male patients and 60% of females.