A 77-year-old Japanese woman who had been treated for follicular lymphoma for 8 years developed abdominal pain and intra-abdominal lymphadenopathies. Colonoscopy revealed an elevated lesion in the rectum, which presented as two humps with erosions. A diagnosis of histologic transformation of follicular lymphoma to diffuse large B-cell lymphoma was made by endoscopic biopsy. This case underscores the importance of endoscopy examinations and biopsy of newly emerged gastrointestinal lesions for the prompt diagnosis of histologic transformation, since salvage chemotherapy must be initiated quickly in such cases.