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Summary.Morphometrie and ultrastruetural studies were performed on biopsy material from the pancreas of an infant with severe leucine-sensitive hypoglycemia and hyperinsulinism, in whom no insulinoma had been found. Qualitatively many large B cells were observed within the pancreatic islets. Quantitatively an about two fold increase of islet tissue proportion (4.2 %) was demonstrated, compared with controls of approximately the same age (1.8%). Differential islet cell counting revealed an increase in A 1 cells whereas the percentage of A 2 and B cells appeared to be unchanged. Ultrastructurally in addition to A, B, and D cells a fourth islet cell type was demonstrated in unusual frequency. Its general function and its particular significance for the hypoglycemic syndrome are unknown. The findings correspond well with recent observations on the islet cell system in cases of neonatal hypoglycemia with leucine-sensitivity and hyperinsulinism.Key words: Neonatal hypoglycemia, hyperinsulinism, leucine-sensitivity, islet hyperplasia, ultrastructure, A, B, D and type IV cells.Reports concerning histopathology [3,4,8,9,16,17,18] or ultrastructure [26] of the pancreatic islets in idiopathic neonatal hypoglycemia are rare. Most of the examined pancreata showed a regular islet system [18], whereas only few exhibited islet hyperplasia. Moreover, as quantitative methods were only rarely applied to islet examination in infants, the diagnosis of islet hyperplasia as a cause of neonatal hypoglycemia remained often doubtful.A recent report of Misugi and coworkers [26] renewed the interest in the islet system in neonatal hypoglycemia. The anthers described islet cell hyperp/asia in three infants with leucine-sensitive hypoglycemia [4] and hyperinsulinism. The ultrastruetural examination revealed a fourth islet cell type in addition to A, B and D cells. The present report of a ease of leucine-sensitive neonatal hypoglycemia confirms the findings of Misugi and associates, and briefly discusses the significance of the observations on the basis of the literature on this subject.