Antibodies to insulin (IA) were measured by graded hemagglutination in 164 patients and in 90 control subjects aged 18-65 years. The greatest prevalence of these antibodies was found in patients with the organic mental syndrome, of whom 70% (n = 110) had an antibody concentration above the normal 95th percentile. Among patients with diabetes, hypertension and cerebrovascular accidents, 42% had a high concentration of IA, while in all other geriatric patients (n = 21) the concentration of these antibodies was within the normal range.