Inflammatory bowel diseases, (IBD) including Crohn's disease and ulcerative colitis, are chronic systemic disorders associated with intestinal and other systemic features. Common hematological manifestations of IBD include: anemia, hypercoagulable state, leukocytosis and thrombocytosis. Recently it has been recognized that lymphoma and leukemia can also be associated with both Crohn's disease and ulcerative colitis. Careful review of the literature reveals more than 30 cases of leukemia reported in patients with IBD. Epidemiological data show that this association is statistically significant (relative risk of 5.3; p < 0.01; 95% confidence interval). This review attempts to characterize the clinical features of this association. The etiology of leukemia in these cases seems to be multifactorial and may involve: genetic susceptibility, environmental factors, immune abnormalities, prior exposure to diagnostic radiation and a variety of therapeutic modalities. No definitive time interval, specific pattern or correlation between the extent of bowel involvement and type of leukemia was found. Treatment of leukemia might affect the activity and severity of the inflammatory bowel disease, but there is very little data relating to this issue. There is some evidence suggesting that bone marrow transplantation performed for leukemia may induce remission of both diseases. Further investigation is still required in the future in order to establish a definite relationship between these two disorders.