Mesalazine, or 5-aminosalicylic acid, is an anti-inflammatory drug frequently used in patients suffering from chronic inflammatory bowel disease, and is generally well tolerated. Its most common side effects are mainly limited to gastrointestinal disorders, headaches, joint pain and skin rashes, and are generally modest and transient. Serious hematological toxicity is very rare (<1/10000), but cases of thrombocytopenia, aplastic anemia, pancytopenia and leukopenia/agranulocytosis have been reported. We report here the case of a patient with inflammatory phenotype colonic Crohn's disease on mesalazine 3g/day who presented with initial leukopenia aggravated by the development of pancytopenia.