Persistent hyperinsulinemic hypoglycemia of Infancy (PHHI) is characterized by episodes of severe hypoglycemia exposing the child to serious neurological sequelae. The morphologic pancreatic anomalies observed consist in either a focal lesion which can be totally cured by a selective surgical resection, or a diffuse and poorly understood lesion of pancreatic tissue, which may appear at first glance to be both macroscopically and microscopically normal, the treatment of which, requires a near-total pancreatectomy. The studies presented concentrate on several pathogenic hypotheses of this latter form of PHHI. We first demonstrate that nesidioblastosis, long considered as the original lesion for the diffuse form of hyperinsulinism, does indeed exist although it is not specific to this disease and does not correspond to a permanent proliferation of endocrine cells. We then dismiss the hypothesis of an enhanced B cell mass, as the volume density of B cells is not systematically increased in hyperinsulinemic children compared to controls. The third hypothesis looking to a decreased D cell mass has to be approached with circumspection as the difference in volume density of somatostatin cells, which tends to be slightly lower in hypoglycemic children, is small and inconsistent. The fourth hypothesis concerns an intrinsic functional lesion of the B cells, and is supported by an increased quantity of proinsulin detected in the Golgi area as well as by the observation of voluminous nuclei and a particularly abundant cytoplasm in certain B cells. These histological anomalies are detected on per-operative frozen sections and guide the surgical treatment.