During a period of 12 months all diabetics treated with insulin or oral hypoglycemic agents (OHA) were registered by collection of prescriptions on the island of Falster, Denmark, with 44,498 inhabitants. The overall prevalence of diabetes mellitus was 12.4 per 1,000 (553 diabetics). Insulin-treated diabetics comprised 231 persons (prevalence rate 5.2 per 1,000) and OHA-treated diabetics comprised 322 persons (prevalence rate 7.2 per 1,000). The prevalence of insulin-treated diabetes especially with an age less than 30 yr was significantly higher than the prevalence in 2 other areas in Denmark. 50% of the insulin-treated diabetics with an onset age greater than or equal to 30 yr were overweight in contrast to 6% of insulin-treated diabetics with an onset age less than 30 yr (p less than 0.01). Among OHA-treated diabetics, 65% were overweight.