A prevalence survey for known diabetes was conducted in a geographically defined population of nearly 40,100 in Oxford in April 1982. The age-adjusted prevalence rate was 10.4/1000 which did not differ significantly from age-adjusted rates of 9.5/1000 in Poole and 10.5/1000 in Southall. The prevalence increased with age and was higher in men than women over the age of 30 years. Our results confirm that there has been a change in the male to female sex ratio and suggest that there are about 500,000 diagnosed diabetics in England and Wales including about 190,000 insulin-treated patients. These findings are consistent with a secular increase in the prevalence of diagnosed diabetes over the last two decades which has important implications for the planning and provision of resources for care.