Five cases of persistent neonatal hyperinsulinemic hypoglycemia (PNHH) were studied by a combined immunofluorescent and Feulgen technique under a computerized microscope which was used to measure the area and absorbance of beta cell nuclei. An increase of 16% was found in the nuclear area and absorbance of the beta cells in PNHH cases, in comparison with age-matched control cases. A smaller increase (9%) was found in nuclear area and absorbance of non-endocrine ductal cells in PNHH cases. In all PNHH and control cases, higher values of nuclear area and absorbance were found for beta cells in the islets of Langerhans than for nesidioblasts. The increase in average size of beta cell nuclei in PNHH can be used as a morphologic criterion for diagnosis in these cases. The increased absorbance of the nuclei stained with the Feulgen reaction is the morphologic expression of polyploidy, and has a correlation to the metabolic activity of beta cells in this disorder. We conclude that, in contrast to the normal postnatal developmental process of nesidioblastosis, nesidiodysplasia is a well-defined histologic entity, manifested by an increase of beta cell nuclear size and its DNA content.