Two boys and one girl developed persisting hypoglycaemia 12, 24, and 48 hours after birth. Although there was no known history of hereditary diabetes mellitus, the birth weight was high in two cases, and some additional traits of foetopathia diabetica could not be excluded clinically. All had high serum insulin levels with frank hyperinsulinaemia in one case. Glucose tolerance tests also indicated hyperinsulinism. Diazoxide (8 to 27 mg/kg) elevated the blood glucose levels, but did not prevent severe hypoglycaemic episodes. The effects of subtotal pancreatectomy were only transient. The patients have now been kept on permanent diazoxide therapy for 2-4 years at dosages slightly lower than those used before operation. No islet-cell tumour was found at the subtotal pancreatectomy. In all 3 cases, the pancreatic islets were markedly hyperplastic and of irregular shape with the occurrence of large B-cells with giant hyperchromatic nuclei and chromophobe "agranular" or sparsely granulated cells. The predominating kind of islet cells showed tinctorial features of A2-cells but--in the absence of available material set aside for ultrastructural analysis--it could not be settled whether this was a result of a proliferation of A2-cells only or of "type IV cells" as well. Against the background of previously published reports, the present cases serve to illustrate that additional accuracy of diagnosis and classification of neonatal persistent hypoglycaemia requires quantitative information about the structural changes in the pancreatic islet cells, and that this can be obtained from conventional biopsy specimens.