Two sexually mature marmosets (Callithrix jacchus) showing clinical signs similar to those seen in wasting marmoset syndrome (weight loss, decreased muscle mass, and alopecia) were evaluated for clinical and anatomic pathologic changes. The most prominent clinical pathologic alterations included macrocytic normochromic anemia, hypoproteinemia, hypoalbuminemia, elevated serum aspartate aminotransferase and alkaline phosphatase levels, and previously unreported changes of thrombocytosis. The principal gross and histopathologic finding was chronic colitis, which appeared to be the most important contributing factor to the development of wasting syndrome in these marmosets.