Chronic idiopathic large bowel diarrhea (CILBD) is a descriptive diagnosis reached by exclusion of the most common diseases associated with chronic large bowel diarrhea. The most important features of irritable bowel syndrome in people are reviewed. The authors report 19 new cases of CILBD that they compare with 37 cases from the literature. The disease appears to be common among dogs referred for colonoscopy. Many dogs with CILBD experience behavioral problems or are exposed to a stressful environment. Most dogs respond to diet change and fiber supplementation. Some benefit from behavior-modifying therapy.