The prevalence of diabetes mellitus was investigated in a northern Finnish community population aged 70 years or over. Of the eligible 483 persons, 78.5% (n = 379) took part in the study. The presence of diabetes mellitus was assessed by questions about the participants' previously diagnosed diabetes and 2 h oral glucose tolerance tests, which were performed according to the current WHO criteria. Only the participants who were on oral hypoglycaemic drugs or insulin treatment were excluded from the 2 h oral glucose tolerance tests. In the total population the prevalence of Type 2 diabetes was 22.0% among men and 28.2% among women; the difference between sexes was not significant (women's risk ratio (RR) 1.3, 95% confidence interval (CI) 0.9-1.9). Among those aged 80 years or over the risk ratio for women was 11.3 compared with men (95% CI 1.6-79.5). Among men the prevalence was higher in the age group 70-79 years compared with those aged 80 years or over (RR 8.1, 95% CI 1.2-57.1). By contrast, among women, diabetes was less common among those aged 70-79 years compared with those aged 80 years or over (RR 0.6, 95% CI 0.4-1.0). The proportion of undiagnosed diabetes was clearly over one-third among men and just over one-third among women. Of men, 31.9% suffered from impaired glucose tolerance; the corresponding figure for women was 35.3%. The comparatively high proportion of undiagnosed diabetes and impaired glucose tolerance among elderly people lends support to the more frequent use of the 2 h glucose tolerance test in clinical practice.