Lymphoma complicating inflammatory bowel disease is well described. Whether the risk of lymphoma is increased by immunosuppressive treatment with azathioprine, 6-mercaptopurine or infliximab is a common concern among patients and physicians considering using these agents. This review aims to quantify the lymphoma risk in inflammatory bowel disease and the added risk attributable to these treatments. The evidence from published cases is that lymphomas occur at sites of active inflammatory bowel disease more often than expected for this to be a chance association. Studies on inflammatory bowel disease populations are conflicting, with some follow-up studies from large inflammatory bowel disease clinics showing an increase in lymphoma incidence, while other population-based studies show little or no increase in risk of lymphoma. A small increase in lymphoma risk in inflammatory bowel disease, perhaps 2-3-fold, may be compatible with both sets of data. Studies of the risks associated with immuno- suppression are less satisfactory, with smaller numbers of patients and relatively short follow-up. The available evidence would support a further increase in lymphoma risk associated with immunosuppressive treatment in inflammatory bowel disease of around fivefold compared to no immunosuppressive use, and tenfold compared to the general population. The risks appear to be less than that associated with renal and hepatic transplant-related immunosuppression. Infliximab treatment is still too new to make a full assessment of its long-term safety, but post-marketing surveillance currently suggests that lymphoma risk may not be any greater than that associated with azathioprine and 6-mercaptopurine. Population-wide surveillance for lymphoma in inflammatory bowel disease would be required to narrow the confidence intervals on these estimates of lymphoma risk in inflammatory bowel disease and immunosuppressive treatment.