Maturity onset diabetes (MOD) is characterized by the fact that the response of insulin secretion to glucose loading is either completely missing or is reduced if compared with that of persons whose metabolism is intact. But insulin secretion can be provoked by other specific stimuli. However, the quantitative IRI response can provide no information as to which of the MO diabetics must be treated by dietetic measures only and which of them are liable to treatment with sulfonylurea. It was, therefore, investigated whether in 109 patients suffering from recently developed overt MOD a differentiation from a therapeutical point of view can be attained by joint evaluation of stimulated IRI secretion, of glucose tolerance, of the dynamics of free fatty acids, of the fasting values of triglycerides and cholesterol and of the body weight. The findings suggest that no better differentiation for the two methods of treatment of MOD stated above is possible by simultaneous evaluation of the parameters of fat metabolism, glucose level and IRI secretion than by IRI secretion and carbohydrate tolerance alone.