Large intestinal disease, and more especially colitis, is a commonly seen problem in small animal practice. Although colitis is most frequently diagnosed in dogs, it is becoming increasing common in cats. The etiology of colitis is not known, but there is general agreement that an immune-mediated response to luminal antigen is involved. In particular, parasites, bacteria and dietary factors may be involved. In approximately 10% of dogs presented with typical signs of colitis, no pathologic lesion will be found on investigation. These dogs have a functional diarrhea associated with some stress factor and are thought to have irritable bowel syndrome (IBS). This condition is most frequently observed in working dogs, although highly nervous and excitable dogs may also exhibit similar clinical signs. Until the underlying etiology of colitis is determined, treatment regimens will remain symptomatic. Recent studies have placed considerable importance on the value of diet in the prevention, immediate and long-term therapy of colitis in dogs and cats. In particular the value of "novel" protein diets, fermentable fiber and polyunsaturated fatty acids is receiving the most attention. It is now possible to maintain patients in long-term remission and to modify the severity and chronicity of colitis by using diet alone. This paper will review the subject of dietary management of colitis and IBS and present results from the author's clinical research program.