Patients with a severe inflammatory bowel disease or who have corticodependance should be treated by thiopurine drugs, azathioprine and 6-mercaptopurine. Although the drugs are usually well tolerated, adverse reactions can occur like bone marrow suppression which is dose-dependent and a delayed side effect requiring regular complete blood count monitoring. We describe a patient with ulcerative colitis who developed azathioprine-induced pancytopenia