A case of hyperinsulinism occuring in a newborn, with a birthweight of 4,050 g, is reported. The hypoglycaemia was refractory to the usual therapy (increase of glucose administration per os, and I.V., corticosteroids, glucagon, diazoxide). At surgery, undertaken at 9 days of age, an adenomatous nodule was removed along with the left part of the pancreas. Death occurred at 18 days, after the child had developed a transitory acidoketosic diabetes and an encephalopathy. Measurement of insulin by radio-immunoassay revealed a strong increase in the ratio insulin/glycaemia, characteristic of nesidioblastoma, as well as a high concentration of insulin in the tumor as compared to normal tissue. On the ultrastructural level, the observed features differed from those seen in children and adults and showed an abnormal overload of dense deposits in the cytoplasm of some histiocytes.