A 36-yr-old man was admitted to our hospital complaining of severe vomiting and diarrhea. Upper gastrointestinal series showed deformity of the duodenum. Serum gastrin level was very high (1,829 pg/ml) and secretin provocative test presented a positive result (peak 4,535 pg/ml). We diagnosed his illness as Zollinger-Ellison syndrome, but failed to identify the site of tumor. His symptoms were controlled with cimetidine 1,600 mg qid, but serum gastrin level was increasing. A year after the first admission, computed tomographic scan and selective angiography demonstrated the tumor, and surgery was performed. A solitary 2-cm tumor was noted at the surface of mesenterium of the duodenum. Frozen and paraffin section of the tumor revealed islet cell tumor apparently within a lymph node. Immunohistological examination revealed positive staining for gastrin alone. No other tumors were detectable by inspection and palpation during the operation. After excision of the tumor, serum gastrin and secretion test were normalized, and the patient remains asymptomatic 1 yr after surgery.