The histologic findings are described in 16 subtotal pancreatectomies performed in patients with hyperinsulinemic hypoglycemia over a 17 year period. All patients had nesidioblastosis, defined as the presence of small packets of two to 25 islet cells scattered throughout acinar tissue and separate from islets of Langerhans. The proliferating islet cells were a mixture of beta, alpha, and delta cells. Two histologic subgroups were apparent: Group I patients had diffuse hyperplasia of the islets of Langerhans as well as nesidioblastosis, and group II patients had more subtle nesidioblastosis alone. The patients in groups I and II were remarkably age segregated. All but one of the group I patients were eight months old or younger. Group II patients ranged in age from three to 15 years. The incidence of nesidioblastosis in the patients undergoing pancreatectomy was higher than the incidence in a group of age matched autopsy controls.