The age of onset and the clinical type of diabetes mellitus were evaluated on the basis of a cross-sectional study of medical records of 14 municipal health centers in East Finland. Altogether 281 patients were classified as having insulin-dependent (IDDM) and 2713 as having non-insulin-dependent (NIDDM) diabetes. Nearly all patients diagnosed before the age of 19 had IDDM, but a large proportion (37%) of all diagnoses of IDDM were made after that age. Six percent of all diabetic subjects in the age group 15–19 yr were classified as NIDDM and the proportion increased rapidly in older age groups. Half of the patients with NIDDM were diagnosed over the age of 64.