The study aimed at tracing the population of insulin-treated diabetics living in the Funen County, Denmark (approximately 450000 inhabitants) on 1 July 1973. It was based on a recording of insulin prescriptions among all prescriptions handled by the pharmacies in Funen County during a five-month period. Through information from medical records and public registries the verification of the diagnosis and the identity of the insulin prescription holders were checked, and it is estimated that the completeness of the study material was above 98%. Age- and sex-specific prevalence rates of insulin-treated diabetes mellitus were calculated. The overall prevalence rate for males was 3.6 per 1,000 and that for females 3.3 per 1,000.