Prevalence and incidence of known diabetes mellitus in Finnish subjects aged 30 years or over in 1970-1987 were investigated using a nationwide register of drug recipients and four population surveys. On the basis of the drug-register data, the prevalence of drug-treated diabetes in men increased from 1.5% in 1970 to 2.8% in 1987. In women, the prevalence increased from 2.1% in 1970 to 3.0% in 1979 and declined thereafter slightly to 2.7% in 1987. The increase in the prevalence of drug-treated diabetes was mainly due to the increase of diabetes in the age groups 60 years and older. The prevalence of all known diabetic subjects in the population studies increased from 2.1% in the health examination survey of 1966-1972 (n = 38,676) to 3.5% in the interview survey of 1976 (n = 10,657) and to 4.5% in the health examination survey of 1979-1980 (n = 7,217) but decreased to 3.3% in the interview survey in 1987 (n = 9,522). The increased prevalence of obesity in men and better survival may be important factors contributing to the increasing prevalence rates of diabetes in Finnish subjects aged 30 years and over.