An 8-month-old, female, mixed-breed dog presented to the Iowa State University Veterinary Teaching Hospital with a 1-month history of vomiting and diarrhea. An exploratory laparotomy was performed revealing markedly distended and fluid-filled small and large intestines that were not obstructed. The clinical condition of the dog did not improve subsequent to exploratory surgery, and it was euthanized. At necropsy, both the small and large intestines were distended (approximately 4 cm in diameter) and fluid-filled, and the wall was thin. The abdominal cavity contained approximately 500 ml of a brownish clear fluid. Microscopic lesions of the intestines were confined to the intestinal tunica muscularis and muscularis mucosae and consisted of locally extensive-to-diffuse replacement of the smooth muscle by fibrous tissue and multifocal infiltration by a moderately dense mononuclear inflammatory infiltrate. A unique finding was the presence of similar microscopic lesions in the tunica muscularis of the urinary bladder and stomach.