A 72-year-old man was admitted to our hospital to undergo a novel small-intestinal endoscopic procedure. He had had occasional episodes of hematochezia over a 2-year period, during which he had been hospitalized twice previously. However, numerous investigations, including hematological and biochemical studies, gastroscopy, colonoscopy, computed tomography, scintigraphy, and angiography had failed to detect the source of bleeding in the gastrointestinal tract. On this a dmission,double-balloon enteroscopy was performed and revealed several ulcer scars with localized dilation of the ileum. Histopathological examination of the biopsy specimens reveled no abnormal finding. Partial resection of the ileum was performed to prevent further gastrointestinal bleeding, and histopathological examination of the resected specimen revealed aggregation of atypical lymphocytes, predominantly in the muscularis propria layer. Immunohistochemical examination demonstrated that the tumor cells were positive for CD20 and BCL2, but negative for UCHL1. Based on these findings, the lesion was diagnosed as a marginal-zone B-cell lymphoma of mucosa-associated lymphoid tissue. Eighteen months after surgery, the patient was still in complete remission. A 72-year-old man was admitted to our hospital to undergo a novel small-intestinal endoscopic procedure. He had had occasional episodes of hematochezia over a 2-year period, during which he had been hospitalized twice previously. However, numerous investigations, including hematological and biochemical studies, gastroscopy, colonoscopy, computed tomography, scintigraphy, and angiography had failed to detect the source of bleeding in the gastrointestinal tract. On this a dmission, double-balloon enteroscopy was performed and revealed several ulcer scars with localized dilation of The ileum. Histopathological examination of the biopsy specimens reveled no abnormal finding. Partial resection of the ileum was performed to prevent further gastrointestinal bleeding, and histopathological examination of the resected specimen revealed aggregation of atypical lymphocytes, predominantly in the muscularis propria layer. that the tumor cells were positiv e for CD20 and BCL2, but negative for UCHL1. Based on these findings, the lesion was diagnosed as a marginal-zone B-cell lymphoma of mucosa-associated lymphoid tissue. Eighteen months after surgery, the patient was still in complete remission.