A 52-year-old woman was treated for colorectal diffuse large B-cell lymphoma (DLBCL) after a prolonged treatment period of azathioprine (AZA) and infliximab (IFX) for Crohn's disease (CD). She had been diagnosed as having colonic CD at the age of 24 years and received AZA from age 29. IFX was added at 47 years of age. She experienced massive hematochezia and anal pain at the age of 52 years and was transferred to our hospital. Endoscopic examination revealed a deep rectal ulcer with arterial bleeding. A stoma constructed at the transverse colon for refractory CD relieved her symptoms. Four months later, computed tomography showed increased thickness of the rectal wall. DLBCL was diagnosed from biopsy specimens of the rectum. Treatment with 6 courses of rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisolone along with additional radiation therapy for remnant rectal lymphoma has resulted in complete remission for over 5 years. Although colorectal malignant lymphoma coexisting with active CD was rare and the lesions were difficult to detect, intensive therapy for CD helped in the diagnosis and successful treatment of the patient.