During treatment for inflammatory bowel disease (IBD) 2 men with ulcerative colitis, aged 52 and 38 years, and a 37-year-old man with Crohn's disease developed Epstein-Barr virus (EBV)-related non-Hodgkin's B-cell lymphoma. The first 2 patients underwent proctocolectomy and the use of immunosuppressive agents was discontinued, after which the lymphoma disappeared. The third patient had icterus, hepatosplenomegaly and pancytopenia; he died from multiple organ failure. Azathioprine and 6-mercaptopurine are first choice therapy in the treatment of steroid-refractory IBD. These immunomodulating agents are associated with the development of EBV-positive lymphomas in the setting of solid organ transplantation. This type of lymphoma is a rare complication in IBD, although the incidence in referral centres appears to be increasing. Since azathioprine is an important drug in IBD, there is a need for identification of IBD patients at risk of developing a lymphoma. EBV-DNA in plasma or in faeces may be a candidate tumour marker.