Mature T-cell and natural killer-cell neoplasms account for 10 to 15% of all non-Hodgkin's lymphomas. Of the various subtypes of mature T-cell and NK-cell lymphomas, extranodal NK/T-cell lymphoma, nasal type (nasal type NK/T-L) is relatively more common among Asians including Koreans. Nasal type NK/T-L is an aggressive, Epstein-Barr virus-associated lymphoma with characteristic expression of NK-cell antigen CD56. In this report, we present an unusual case of EBV(), CD56() NK/T-L of oropharynx which recurred in duodenum after the period of complete remission lasting for 10 years. A 58-year-old woman presented with 3 months history of abdominal pain. Gastroduodenoscopy showed the diffuse wall thickening with multiple ulcerations in bulb and proximal second portion of the duodenum. Pathological examination revealed the infiltration of atypical lymphocytes, which was positive for CD3, CD4, CD5, TIA-1, and EBV and was negative for CD15, CD20, and CD56, consistent with NK/T-L of mature T-cell origin. The past medical history included the presence of oropharyngeal mass 10 years earlier, which was diagnosed as polymorphic reticulosis. The mass resolved completely after the radiation therapy, and she remained free of the disease for 10 years. Upon review, the oropharyngeal biopsy showed an identical morphology and immunophenotype with duodenal lesion. In conclusion, we experienced an unusal case of NK/T-cell lymphoma, nasal type recured in the duodenum. (Korean J Gastrointest Endosc 2005;30:204209)