In a newborn severe persistent hypoglycemia due to an insulin-producing tumorous proliferation of pancreatic islet cells (insulinoma) was observed. The insulinoma showed the histologic pattern of focal adenomatosis of islet cells. According to the present literature the focal proliferation of islet cell complexes seems to be a frequent and particular feature of insulinomas in the newborn. Differential islet cell staining identified 80%-90% of the proliferated islet cells as B cells. 10%-20% of the cells were found to be A or D cells. Ultrastructurally the majority of the proliferated islet cells were well differentiated B cells. The remaining cells represented either A or D cells or a fourth islet cell type with small spheric granules. Electronmicrscopic evidence of transitions between differentiated islet cells, particularly B cells, and the fourth islet cell type suggests that the fourth islet cell type might represent a precursor cell within the APUD-cell system.