This paper presents a case of non Hogdikin's lymphoma of the duodenum which was preoperatively diag-nosed. A 66-year-old woman was admitted to the hospital for the evaluation of a duodenal tumor, which was incidentally detected by routine examination of GI upper series. Duodenography demonstrated a giant whole circumferential elevated lesion with crater in the upper duodenal flexure. Endoscopic examination of the duodenum showed the whitish irregular elevated lesion. Biopsy specimens taken from the lesion revealed diffuse growth of medium sized atypical lymphoid cells, suspected non Hodgikin's lymphoma (NHL). Ultrasonography and CT scan showed a thickness of the duodenal wall which was suspected of invasion to the pancreas. Pancreatico-duodenectomy with lymphnodes dissection and reconstruction by means of Child II method was performed. Histologically the tumor was diagnosed as NHL (diffuse medium sized B cell type) spread involving the subserosa. Adjuvant chemotherapy (VEPA) was performed postoperatively. No recurrence has oc-cured 12 months after the operation. Primary malignant lymphoma of the duodenum is relatively rare and there have been only 57 cases (including our case) diagnosed before operation.