A 38-year-old female was admitted for investigation of the cause of hypergastrinemia, hypercalcemia and an elevated plasma parathyroid hormone (PTH) level. Her siblings, elder brother and sister who had duodenal carcinoid tumor with hypergastrinemia and parathyroid adenomas, were diagnosed as multiple endocrine neoplasia (MEN) type 1. In the present patient, endoscopic examination showed a carcinoid tumor of the duodenum, but examinations by computed tomography (CT) and ultrasonography of the abdomen failed to reveal pancreatic lesions. Serum gastrin levels of this patient and her siblings improved to the normal level after resection of carcinoid tumors. The hypergastrinemia accompanying MEN type 1 in these cases might be due to carcinoid tumor of the duodenum.