The medical records of 37 dogs diagnosed with chronic idiopathic large-bowel diarrhea were reviewed. The median age of affected dogs was 6 years. The median body weight was 13.9 kg. The median duration of clinical signs before diagnosis was 32 weeks. Diarrhea usually was intermittent and characterized by increasing frequency, fecal mucus, hematochezia, and tenesmus. Vomiting was common but usually much less frequent and severe than the diarrhea. A variety of stressful factors and abnormal personality traits were identified. CBC and serum biochemistry usually were normal. Fecal examination rarely identified parasites. Rectal cytology specimens were most often normal, but some dogs had increased numbers of neutrophils. The colonic mucosa often was normal during colonoscopy, but decreased numbers of lymphoid follicles were found in some dogs. Histopathologic evaluation found that colonic mucosa was within normal limits. Treatment with soluble fiber (Metamucil) added to a highly digestible diet (Hills i/d) resulted in a very good to excellent response in most dogs. The median initial dosage of Metamucil was 2 tablespoons (2 T) per day. In some dogs, the fiber dosage was reduced or eliminated, or a grocery store brand of dog food was substituted, without causing diarrhea to return.