Idiopathic inflammatory bowel disease was the diagnosis for 58 dogs and 26 cats, with signs of persistent gastroenteritis, failed responses to dietary trials, and histologic evidence of cellular infiltrates unrelated to other causes of gastrointestinal tract inflammation. Clinical signs of large intestinal dysfunction, watery diarrhea, vomiting, and anorexia with weight loss were common. Nonspecific hematologic, biochemical, and radiographic abnormalities frequently were observed. Mucosal biopsy specimens, obtained endoscopically, were histologically evaluated for severity of mucosal epithelial damage. Mucosal erythema, friability, enhanced granularity, and ulceration or erosion were the predominant endoscopic lesions. Inflammatory bowel disease lesions of moderate severity predominated in the stomach, duodenum, and colon. Lymphocytic/plasmacytic infiltrates were limited to the lamina propria in biopsy specimens from all regions of the gastrointestinal tract. Inflammatory bowel disease commonly is associated with chronic gastroenteritis in dogs and cats.