Nesidioblastosis is a well-recognized cause of persistent hypoglycaemia in neonates. We describe the case of a 24-year-old woman who presented with hyperinsulinaemic hypoglycaemia in whom an insulinoma could not be identified at operation which resulted in her undergoing a subtotal distal pancreatectomy. Histological examination revealed the presence of nesidioblastosis. The finding of nesidioblastosis in adults has been reported previously, albeit rarely. We report the use of in situ hybridization to characterize the patterns of insulin and proglucagon gene expression in the resected pancreas. Insulin expression was observed in both the islets and also the isolated nesidioblasts. The alpha-cells of the islets had lost their usual peripheral distribution suggesting that not only is insulin gene expression dysregulated but that the islets are structurally abnormal.