Thiopurine drugs, such as thioguanine, mercaptopurine and azathioprine, are used for treating inflammatory bowel disease, such as ulcerative colitis. One must be aware of the serious side effects these drugs can have (e.g. bone marrow depression). A 56-year-old man with ulcerative colitis was treated with mercaptopurine. He developed leukopenia as a result. Thioguanine was started ten months later, resulting in life-threatening pancytopenia. Thiopurine methyltransferase (TPMT) genotyping proved that the patient was a poor metaboliser of thioguanine (TPMT3A*/3A*). Dose reduction is recommended for patients with reduced or absent TPMT activity. This life-threatening side effect could have been prevented by taking a number of relatively simple precautions.