The peptide C, polypeptide secreted by the pancreas at the same time as insulin, presents a great interest in the evaluation of diabetic patients. First it allows a differentiation between insulin dependent diabetes (IDD) and non insulin dependent (NDD). A low and non stimulated levels of peptide C signifies an insulin dependence. Within the group of IDD patients the peptide C was low when the diabetes was discovered at a younger age and its secretion diminished as the diabetes progresses. The peptide C has also a prognostic interest in IDD. Low and non stimulable levels of peptide C signifies a difficult control of diabetes which needs two injections per day while high and stimulable levels will be seen in diabetes easy to control with one injection of insulin. Finally, values of peptide C does not permit to predict the onset of diabetic complications (retinopathy, acidocetosis) as well as the control of patients.