Background: Collagenous colitis is a chronic inflammatory bowel disease of unknown etiology. It is fairly common in adult humans, but rare in infants, and has been associated with autoimmune disorders. Methods: We report four infant baboons (age 7-12 months) that had received a transplant at 3 months of age and subsequent immunosuppressive therapy for periods of 4-10 months. All presented identical symptoms within a period of 4 weeks, including weight loss associated with chronic watery diarrhea that was unresponsive to standard antimicrobial treatment. Results: Clinical chemistry evaluations were within normal ranges, viral causes were ruled out, and fecal and blood cultures were repeatedly negative. At necropsy, two infant baboons were found to have a form of collagenous colitis. In the remaining two baboons that had identical clinical features, immunosuppressive therapy was discontinued and treatment with budesonide was initiated. Both baboons recovered and remained well on no medication until the end of follow-up (24 months). Conclusions: Collagenous colitis has occasionally been reported in patients with organ transplants. It has been reported only once previously in baboons. The four cases reported here strongly suggest that 1) clinical features as well as histopathological findings of collagenous colitis in baboons are very similar to those in human patients; 2) it was associated with the immunocompromised state of the baboons, as two nonimmunosuppressed age-matched baboons in close proximity did not develop the condition; and 3) it may have had an infectious origin, as all four cases developed within a 4-week period of time. Copyright © 2012 Crohn's & Colitis Foundation of America, Inc.