Seven surgical specimens of pancreas, obtained at laparotomy from infants suffering from persistent hyperinsulinemic hypoglycemia, were analyzed by qualitative and quantitative immunocytochemistry and by electron microscopy. In five cases a multifocal ductuloinsular proliferation, in one a focal adenomatosis, and in one a solitary encapsulated nodule (adenoma) were observed. Combinations of the different patterns of proliferation were seen in six cases. Budding off from the ductular epithelium and interposition of endocrine cells between ductular epithelial cells were prominent features common to all cases. An almost fivefold increase of the mean total area occupied by endocrine tissue was found over that of age-matched controls. Four cell types were seen to participate regularly in the proliferation, and their ratios were remarkably constant in all cases, mean figures being 62:21:9:8 per cent for B:A:D:D₁ cells, respectively. The ratio of B cells per total endocrine area in nesidioblastosis was very close to that per islet of the controls (62:59 per cent).

Since common features were found in all or in the majority of cases, it is suggested that the various patterns of proliferation are merely morphologic variations of the same basic defect. Nesidioblastosis may result from inappropriately controlled development of the endocrine pancreas that is not arrested but carries on beyond birth and during infancy. The application of specific immunocytochemistry as a necessity for full appreciation of the extent of endocrine proliferation is stressed.