The prevalence of diabetes mellitus was investigated in a sample of people aged 65 to 85 years, using a modified oral glucose tolerance test and 1985 WHO criteria. Of the sample of 861, 52 had previously been diagnosed diabetic; 583 consented to be tested and 19 were diabetic. The prevalence of previously diagnosed diabetes was 6.0 (95% CI 4.3 to 8.1) %, and the prevalence of previously undiagnosed diabetes was 3.3 (95% CI 2.0 to 5.0) %. The high prevalence of previously diagnosed diabetes might be due to the longstanding community diabetes care in the area studied.