For the purpose of studying the recent changes in the mortality from diabetes mellitus the crude death rates and the sex-age-specific death rates from diabetes mellitus obtained from Vital Statistics in Japan and the prevalences of diabetes mellitus obtained from Patients Investigation in Japan were analysed. The age-adjusted death rates were calculated from 1950 to 1974 by means of direct method on the basis of sex-specific population in Japan in 1935. 1) The crude death rates and the age-adjusted death rates from diabetes mellitus showed some increases by years, and those death rates of females were superiorr to those of males. 2) The death rates from diabetes mellitus of young people (15-29 years of age) had tendencies to decrease by years, on the contrary those of old people had tendencies to increase by years. The age-specific death rates of females were superior to those of males till the age of 34, but after the age of 35 the death rates of males were superior to those of females. 3) The average sex-age-specific death rates in 1950-54 had a step at the age of 20-29 years, and this disappeared almost in 1970-74. 4) The prevalences of diabetes mellitus from 1965 to 1973 showed increases at the age of 25-64 years, but those increases were not recognized at the age of 20-24 years. The prevalences of diabetes mellitus of males were superior to those of females. The death rate-prevalence ratios of diabetes mellitus showed some decreases at the age of 25-64 years, but those decreases were not recognized at the age of 20-24 years.