Mantle cell lymphoma (MCL) comprises 2.5%-7% of all non-Hodgkin's lymphomas, and the gastrointestinal tract is involved in about 20% of cases. Multiple lymphomatous polyposis (MLP) is an uncommon disease that is regarded as the intestinal form of MCL. We present a rare case of gastrointestinal MCL without MLP, and demonstrate that rituximab was effective for the treatment of this patient. A 61-year-old man presented with continuous diarrhea and hematochezia for a period of 5 months. Superficial lymph nodes were not palpable, but both tonsilla were enlarged. The level of soluble interleukin (IL)2-receptor was 3480 U/ml (normal <500 U/ml). Colonoscopy showed diffuse redness with erosion, without observation of any venous capillary, with these findings continuing from the rectum to the ileum. Upper gastrointestinal endoscopy showed a slightly rough gastric mucosal surface, and chicken-skin like mucosa was observed in the second portion of the duodenum. Small-to-medium size lymphoma cells were seen histologically from the tonsilla to the rectum. The lymphoma cells were immunohistochemically positive for CD5, CD20, CD79a, and cyclin D1. Polymerase chain reaction analysis revealed a chromosomal translocation t(11;14)(q13;q32) in the bcl-1 gene. We diagnosed this as a case of MCL from these findings. For treatment, the patient received a total of ten courses of combination chemotherapy consisting of cyclophosphamide (1000 mg), doxorubicin (70 mg), vincristine (2 mg) and prednisolone (50 mg) (CHOP), which led to a partial remission. However, 2.5 years later, massive infiltrations of the lymphoma cells were found in the colon and stomach. As the infiltrating lymphoma cells expressed CD20 molecules on their surfaces, the patient was treated with a chimeric anti-CD20 monoclohal antibody, rituximab, which showed significant efficacy, and a second partial remission was achieved.