Congenital hyperinsulinism is clinically characterized by an inappropriate insulin secretion resulting in recurrent severe hypoglycemia. Nesidioblastosis, the proliferation of islet cells budding off from ducts, has been considered for years as the histologic lesion responsible for the syndrome. In our morphologic studies, we demonstrate that nesidioblastosis is not specific of the disease, which is actually not a single entity. Indeed, we recognize the existence of 2 different forms-a diffuse form and a focal form-and demonstrate that they can be differentiated by morphologic criteria, even on frozen sections during surgery. This histologic distinction directs the therapeutic approach because the patients experiencing the focal form of the syndrome can be completely cured by a very limited pancreatectomy. Molecular findings confirmed the reliability of this histologic distinction, showing a specific background for each form.