Studies have suggested a possible association between inflammatory bowel disease (IBD) and the Philadelphia-negative chronic myeloproliferative neoplasms (MPNs). The mechanisms behind this association have not been investigated yet, but in this review, we find it most likely to involve complex interactions between genetic, treatment-related and inflammation- and immune-mediated factors. When patients with IBD present with persistent leukocytosis and/or thrombocytosis, it may reflect concomitant MPN, and early detection and treatment of MPNs may prevent some of the complications related to these diseases.