The cause of mucosal injury in inflammatory bowel disease (IBD) is not clear but likely involves infectious agents or other toxins followed by an abnormal immune response in genetically susceptible individuals. The inflammatory cytokines appear to play a key role in both the susceptibility of some individuals and the tissue damage that accompanies IBD. The generation of transgenic and gene-targeted (knockout) animals has provided invaluable information regarding the cytokines and cellular immune effectors that are important in IBD. Information from these and other preclinical animal models, such as those involving interleukin 11, has led to human trials testing novel therapies for IBD and other diseases in which inflammation of the gut mucosa is an important component. Thus, expression of inflammatory cytokines appears to be an important target for the development of novel therapies for IBD and other diseases in which intestinal mucosal damage occurs, such as mucositis and graft-versus-host disease.