Ninety-eight patients with diabetes mellitus (52 women and 46 men) underwent a glucagon stimulated C-peptide test. Treatment was instigated on the basis of classical criteria, independent of the test results, and remained unchanged during a 2-yr test period. The type of treatment and metabolic control were then related to the test results. Fasting C-peptide concentrations do not discriminate between insulin dependent and non-insulin dependent diabetes mellitus. C-peptide concentrations 10 min after intravenous injection of 1 mg of glucagon, however, correctly classified the patients. With the help of this test, the correct model of treatment for patients with diabetes mellitus can be chosen.