Primary colorectal lymphoma is a rare form of presentation of gastrointestinal tract lymphomas. Inflammatory bowel disease and its treatment are risk factors for its development. We report a 47-year-old male patient with Ulcerative Colitis of two years of evolution, treated initially with azathioprine and later on with infliximab. Due to a relapse in symptoms after the second dose of infliximab, a new coloncoscopy was performed showing a rectal ulcerative lesion, corresponding to a large cell Non-Hodgkin's Lymphoma. The patient was successfully treated with RCHOP chemotherapy (Rituximab cyclophosphamide doxorubicin vincristine prednisone). He is currently in disease remission.