Insulin is synthesized in the ß-cells of the islets of Langerhans as a precursor molecule, proinsulin. In the secretory granules of the ß-cells, proinsulin is cleaved into insulin and into a 31 amino-acid peptide, called the Connecting Peptide or C-peptide. Insulin and C-peptide are secreted in equimolecular amounts. However, because of its longer half-life, the plasma concentration of C-peptide is higher than that of insulin. The determination of plasma C-peptide allows an assessment of the endogenous insulin production, even in the presence of exogenous insulin administration or in the presence of circulating anti-insulin antibodies.Moreover, the determination of C-peptide in urine provides a reliable index of the insulin production when blood sampling is difficult or when an integrated estimation of C-peptide secretion over a period of several hours is requested.