Laidlaw coined the term nesidioblastosis in 1938 to characterize the neodifferentiation of the islet cells of Langerhans from pancreatic duct epithelium. It is well recognized in the pediatric population as a frequent cause of persistent neonatal hypoglycemia. However, its occurrence in adults is presumed to be rare and, therefore, it is not appreciated as a cause of hyperinsulinism. Three women, aged 29, 42, and 63, with adult onset hyperinsulinism secondary to nesidioblastosis are reported. All three patients required near-total pancreatectomy. The preoperative findings were consistent with hyperinsulinemic hypoglycemia as with insulinomas. Results of pancreatic imaging studies were normal in two patients and one patient had a pancreatic examination by computerized tomography and magnetic resonance imaging, with false-positive results. Two of the three patients had previously undergone a 50 per cent distal pancreatectomy in which the resected specimens were interpreted as normal in one patient and consistent with nesidioblastosis in the second. Both patients subsequently developed recurrent symptomatic hyperinsulinemic hypoglycemia that persisted despite dosage adjustments in diazoxide therapy. The oldest patient underwent a 95 per cent pancreatectomy at the initial surgical exploration because an insulinoma could not be identified. The other patients underwent a completion 95 per cent pancreatectomy. In both, histochemical examination of each specimen disclosed nesidioblastosis, characterized by clusters of islet cells interspersed throughout the exocrine tissue.