We report the clinical characteristics of all insulin-treated diabetic patients (783 males and 716 females) living on July 1 1973 in a Danish county. In 18 patients diabetes was believed to be secondary to another disease or condition. Of all patients, 43% of the males and 55% of the females were greater than age 50 years and 42% and 36% of males and females, respectively, had a disease duration of less than 10 years. The overall proportion of patients treated permanently with insulin from diagnosis was 85% and 79% in males and females, respectively, but within the subgroup of patients with current age less than 30 years these proportions were 99% and 96%, respectively. Statistical analysis indicated that insulin-treated diabetes is a clinically heterogeneous group of diseases; thus, low age at onset combined with immediate and permanent insulin treatment delineate one subtype, while relatively high age at onset and late start and/or interruption of insulin treatment delineate other subtype(s). It is estimated that the proportion of patients with truly Type 1 (insulin-dependent) diabetes is greater than 50% in this cross-sectional population of insulin-treated patients.