The epidemiology, mortality, and use of hospital services of insulin-treated diabetes mellitus have been studied in Fyn County, Denmark (450,000 inhabitants). The overall prevalence rate was 3.6 and 3.3 per 1000 for men and women, respectively, and overall annual incidence rates have been estimated to be 15 and 11 per 100,000, respectively. Based on a mortality analysis, it is estimated that the overall mortality rate in this patient population is 4-5% per year. During follow-up, hospital admission rates were estimated to be 0.47 and 0.53 per patient person-yr for men and women, respectively; corresponding values for estimated number of hospital bed days used by the patients were 7.2 and 9.6 per person-yr. Only 26% of the patients (74% in the age class 0-9 yr) attended a diabetes outpatient clinic at the prevalence date (1 July 1973); the remaining patients were supervised by their general practitioner. Based on these figures, an epidemiologic model for insulin-treated diabetes has been formulated, and it is concluded that this disease represents a major public health problem.