Several forms of primary and secondary hematological malignancies were rarely observed during the clinical course of inflammatory bowel diseases (IBD). Patients needing a prolonged treatment with immunosuppressants, such as azathioprine or methotrexate, with familiarity and genetic predisposition seem to be at a higher risk of leukemia. On the other hand, asthenia, thickness, and fever may be the symptoms of the onset of each kind of hematological malignancy. The finding of anemia, alteration of leukocyte count and large undetermined cells may suggest increased probability of abnormal proliferation of a single white blood cell line. In this report, the occurrence of hematological malignancies is described in five patients affected by IBD (three with ulcerative colitis and two with Crohn's disease) attending our Gastroenterology Unit.