All pharmacological interventions, including the use of disease suppressive agents in rheumatology, are associated with adverse side effects in a proportion of patients treated. Adverse events, often occurring early during treatment, will be ascertained during clinical trials and by surveillance studies after marketing. Longer term complications, such as malignancy, and indeed all rare events, are unlikely to be detected until large numbers of patients have been treated. Thus observation needs to be continued beyond the treatment period. Immunosuppressive treatment is considered to be a potential risk factor for both malignancy and life threatening infection. The use of treatments such as azathioprine is associated with an increased risk of lymphoproliferative malignancies in patients with rheumatoid arthritis.1-3 Immunosuppressed patients are also at risk of serious infections—for example, fromMycobacterium tuberculosis, pneumocystis, and even from fungi.4 In current clinical practice these small risks are accepted if the potential benefit for the patient is proportionately greater. Informed prescribing of new agents therefore requires knowledge of the risk of such longer term adverse events.