Nesidioblastosis is rare in adults and accounts for 0.5-5% of cases of organic hyperinsulinemia. The diagnosis of nesidioblastosis should be considered when peroperative imaging modalities fail to localize a lesion in patients with hyperinsulinism. Two female patients, aged 55 and 16 years, with hyperinsulinemic hypoglycemia are reported. Somatostatin receptor scintigraphy showed slight focal activity in both patients. The first patient underwent a Whipple procedure and became diabetic. The second patient underwent a distal hemi-pancreatectomy and suffered from recurrent hypoglycemic episodes 3 months after surgery, for which she is presently being treated with octreotide. Histological examination of the resected pancreata revealed focally increased islet tissue and a number of slightly hypertrophic beta cells. Such histological abnormalities have been related to functional changes of β-cells. In infantile nesidioblastosis, a proportion of cases has been associated with mutations in one of several genes. Whether such mutations, leading to hyperinsulinism, also play a role in adult nesidioblastosis is presently unknown.