Diabet. Med. 29, e138–e141 (2012) Background  Insulinoma is a tumour of insulin‐producing cells of the pancreas and is known to be one of the causes of hypoglycaemia. Usually, appropriate removal of the insulinoma results in normalization of blood glucose levels. However, we found novel cases of insulinoma, in which hyperglycaemia developed soon after resection of the insulinoma. Case report  We encountered two patients with repeated hypoglycaemia caused by insulinoma. Following removal of the insulinoma, unanticipated hyperglycaemia was observed in both patients. Thereafter, their blood tests revealed low levels of serum C‐peptide and high titres of anti‐glutamic acid decarboxylase antibody, indicating concomitant Type 1 diabetes. Indeed, histological examination of the resected specimen revealed that one patient showed insulitis in non‐tumorous pancreatic tissue in which β‐cells had already disappeared. Moreover, inflammatory cells infiltrated the insulinoma, as if it were insulitis of Type 1 diabetes, suggesting the existence of anti‐islet autoimmunity. Conclusion  These are first cases of insulinoma associated with underlying Type 1 diabetes. Physicians should be aware of the possibility that insulinoma may mask Type 1 diabetes, and measurement of anti‐islet autoantibodies may be helpful to find underlying Type 1 diabetes, such as in these cases. It is pathologically interesting that the immune cell infiltration into insulinoma may be suggestive of anti‐islet autoimmunity.