In recent years, a substantial number of baboons died at the Southwest Foundation for Biomedical Research following protracted intractable diarrheas. The histopathologic diagnosis at autopsy was chronic colitis. The diarrhea could last for more than one year and occurred in infants, juveniles or young adult baboons, i.e. < or = 8 years of age (a healthy baboon may live up to 27 years). The aim was to assess the histopathologic subtype of chronic colitis in a relatively large number of colonic specimens obtained at autopsy (n = 132). In 88 out of 132 baboons with chronic diarrhea, the colonic mucosa was well preserved for histological examination. At review, various histological phenotypes of chronic colitis were disclosed in 86 of the 88 baboons. Chronic lymphocytic-plasmacytic colitis was found in 54.6% (47 out of 86), chronic ulcerative colitis in 15.1% (13 out of 86), Crohn's colitis in 12.8% (11 out of 86), superficial lymphocytic colitis in 10.5% (9 out of 86), cryptal lymphocytic colitis in 5.8% (5 out of 86) and collagenous colitis in 1.2% (1 out of 86). Chronic ulcerative colitis, Crohn's colitis, superficial lymphocytic colitis and collagenous colitis in baboons closely mimic corresponding histological phenotypes of chronic colitis in humans. Recently, cryptal lymphocytic colitis has also been found in humans. The awareness that chronic colitis in baboons is not one disease but a series oi chronic inflammatory changes, having common clinical symptoms and similar gross appearance, may lead to the correct diagnosis of the subtype of the disease. Only then would it be feasible to systematically initiate the search for the corresponding etiologic agent(s), aiming to tailor specific therapeutic strategies in those animals.