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Myelosuppression is an important and potentially lethal complication of azathioprine treatment. The blood count has been reviewed in all patients treated with azathioprine for inflammatory bowel disease over 27 years in one hospital. Altogether 739 patients (422 with Crohn's disease, 284 with ulcerative colitis, and 33 with indeterminate colitis) were treated with 2 mg/kg/day azathioprine for a median of 12.5 months (range 0.5-132) between 1964 and 1991. Full blood counts were performed monthly for the duration of treatment. In 37 patients (5%) who developed bone marrow toxicity, the drug was withdrawn or the dose reduced. Thirty two of these patients were asymptomatic and five developed symptoms. Leucopenia (white blood count less than 3.0 x 10g/l) occurred in 28 (3.8%) patients, in nine of whom it was severe (white blood count < 2.0 x 10(9)/l). Of these nine patients, three were pancytopenic: two died from sepsis and the other had pneumonia but recovered. A further two patients with severe leucopenia developed a mild upper respiratory infection only. Thrombocytopenia (platelet count < 100,000 x 10(6)/l) in 15 patients was associated with leucopenia in six and developed in isolation in a further nine (total 2%). Isolated thrombocytopenia was never clinically severe. Myelotoxicity from azathioprine developed at any time during drug treatment (range 2 weeks-11 years after starting the drug) and occurred either suddenly or over several months. Bone marrow suppression as a result of azathioprine treatment is uncommon when a moderate dose is used, but is potentially severe. Leucopenia is the commonest and most important haematological complication. Regular monitoring of the full blood count is recommended during treatment.