We have studied 22 diabetic patients, 14 type I and 8 type II, in order to determine if there is a correlation between metabolic control and pancreatic reserve of insulin. All the patients were treated with optimum doses of bolus/basal insulin. They underwent a peptide C test (at baseline and after 3 stimulus with glucagon) and every month, during 3 months, HbA1c and fructosamine were measured, with monthly self control of glycemia. Both HbA1c and fructosamine showed a statistically significant improvement during the study. In all the cases, there was a negative correlation between metabolic control and pancreatic reserve, with statistical significance for type I, especially regarding the response of peptide C to the administration of glucagon. We conclude that the preservation of a good endogenous secretion of insulin benefits the metabolic control of diabetes.