We report a patient, a 23-year-old man, who had clinical and laborator
y findings suggestive of insulinoma. Although imaging studies did not
reveal any tumors in the pancreas, distal pancreatectomy was performed
because the possibility of small insulinoma could not be completely e
xcluded. Grossly, the surgically removed pancreas did not reveal any t
umors. Microscopically, the pancreas exhibited islet cell hyperplasia
and nesidioblastosis. To our knowledge, this is the first authentic re
ported case of islet-cell hyperplasia occurring in a Japanese adult.