Plasmablastic lymphoma is a very rare and recently-described subtype of diffuse large B-cell lymphoma. It has a poor prognosis despite intensive chemotherapy treatment. A 57-year old woman with perianal Crohn's disease receiving azathioprine and infliximab developed this type of lymphoma after a short period of time on the treatment. She also had a hepatitis C virus chronic infection which had not been diagnosed or treated before. There is no solid scientific evidence that either immunomodulators or anti-TNF drugs have a definitive role in the appearance of malignancies, and therefore there are no clear recommendations to limit their use. In these patients, there are some other factors we have to take into account, like the inflammatory bowel disease in itself and its behaviour over time, or the comorbidities of the patient, with special attention to virus infections. In this case report, we will analyse the role of these factors in the development of lymphoproliferative disorders and the recommendations given by experts to avoid their appearance.