Idiopathic chronic colitis was diagnosed in 13 dogs. Owners sought veterinary care because of semiformed to liquid feces, fresh blood and/or mucus in the feces, tenesmus, increased frequency of defecation, vomiting, weight loss, and flatulence in their dogs. A lymphocytic, plasmacytic infiltration in the colonic lamina propria was found on colonic biopsy specimens. Signs resolved in all 13 dogs after they were fed a low residue, easily assimilated, relatively hypoallergenic diet. In 11 dogs, two commercial diets not previously fed to these dogs were successfully substituted for the initial test diet, without causing recurrence of signs. Only two of these 11 dogs subsequently tolerated a switch to diets that had been fed at the time of onset of signs of colitis. All 13 dogs have been successfully managed from 2 months to 28 months following the initiation of dietary therapy. The results of these dietary challenges strongly suggest a dietary role in the pathogenesis of this disorder, and also illustrate the importance of dietary therapy in the management of idiopathic chronic colitis.