SummaryA case with insulinoma is described. Insulinoma is a secreting adenoma of islet-cells of the pancreas. The diagnosis relies upon clinical observation of hypoglycaemia attacks and upon demonstration of an inappropriate plasma insulin secretion. For this purpose, it appears that measurement of blood glucose and plasma insulin concentrations during an overnight or a prolonged fast is the most valuable test. The tolbutamide test and the insulin-induced hypoglycaemia with plasma C-peptide measurement may be helpful to improve the diagnosis. Selective arteriography i.s useful to locate the tumor(s).