A 73-year-old Japanese woman was hospitalized for detailed examination of nausea, diarrhea and loss of appetite. Atypical erosion in the ileum was found on endoscopy. Biopsy of this erosion showed proliferation of cells containing numerous Russell bodies. Differential diagnoses considered were Russell body enteritis, crystal-storing histiocytosis, Mott cell tumor, immunoproliferative small intestinal disease(IPSID) and mucosaassociated lymphoid tissue(MALT) lymphoma. The cells containing prominent Russell bodies showed diffuse positivity for CD79 a and CD138, but negative results for CD20, CD3, UCHL-1, CD38 and CD68. Russell bodies were diffusely positive for lambda light chain, but negative for kappa light chain, and immunoglobulin(Ig)G, Ig A and Ig M. Based on these findings, Russell body enteritis, crystal-storing histiocytosis and IPSID were ruled out. As the tumor formed no mass lesions and was restricted to the gastrointestinal tract, MALT lymphoma with extensive plasma cell differentiation was finally diagnosed. The patient showed an unexpectedly aggressive clinical course. The number of atypical lymphocytes in peripheral blood gradually increased and T-prolymphocytic leukemia(T-PLL) emerged. The patient died of T-PLL 7 mo after admission. Autopsy was not permitted. A 73-year-old Japanese woman was hospitalized for detailed examination of nausea, diarrhea and loss of appetite. Atypical erosion in the ileum was found on endoscopy. Biopsy of this erosion showed proliferation of cells containing numerous Russell bodies. Differential diagnoses considered were Russell Body enteritis, crystal-deposition histiocytosis, Mott cell tumor, immunoproliferative small intestinal disease (IPSID) and mucosaassociated lymphoid tissue (MALT) lymphoma. The cells containing prominent Russell bodies showed diffuse positivity for CD79 a and CD138, but negative results for CD20, CD3 , UCHL-1, CD38 and CD68. Russell bodies were diffusely positive for lambda light chain, but negative for kappa light chain, and immunoglobulin (Ig) G, Ig A and Ig M. Based on these findings, Russell body enteritis, crystal- As the tumor formed no mass lesions and was restricted to the gastrointestinal tract, MALT lymphoma with extensive plasma cell diff The patient showed an unexpectedly aggressive clinical course. The number showed atypical lymphocytes in peripheral blood increased increased and T-prolymphocytic leukemia (T-PLL) emerged. The patient died of T-PLL 7 mo after admission. Autopsy was not permitted.