A case of suspected clinically hormonally active insulinoma in a 48-year-old woman is presented. Despite the lack of features, which might correspond to the insulinoma in radiological examinations, the patient was qualified for a distal subtotal pancreatectomy and then, due to persistent hyperinsulinism, for total pancreatectomy. The insulinoma was found neither in a palpable examination of the pancreas nor in the intraoperative ultrasonic examination. In a histopathological examination supplemented with immunohistochemical tests, nesidioblastosis - a rare cause of hypoglycaemia in adults - was diagnosed.