Studies of diversion colitis have not shown a consistent pattern of histopathologic features, and many descriptions are difficult to interpret because of the presence of underlying intestinal mucosal disease. To define the histologic changes in patients free of other mucosal inflammatory disease, we studied the resected segments of bypassed colorectum from 37 patients with Hirschsprung's disease treated by a two-stage procedure, using rectal biopsy specimens taken for initial diagnosis and trimmings from proximal to the stoma as controls. Biopsy specimens from a further 14 patients of similar age but without colorectal mucosal disease were used as additional controls. The histology of the bypassed segment was abnormal in all patients. Twenty-six had diversion colitis characterized by diffuse follicular lymphoid hyperplasia; lamina propria expansion by plasma cells, lymphocytes, and some neutrophils; cryptitis; reactive epithelium; and mucin depletion. Crypt abscesses, aphthous ulcers, mild architectural distortion, and Paneth cell metaplasia were noted in more severe cases. The remaining 11 patients had mild follicular lymphoid hyperplasia and an increase in lymphoplasmacytic infiltrates, with absence of neutrophils, epithelial injury, and other changes seen in diversion colitis, a pattern we term "diversion reaction." Diversion colitis is common in children with a bypassed colorectum. It can be distinguished histologically from other mucosal diseases in most cases. We hypothesize that diversion reaction may be an inevitable consequence of colonocyte nutrient deficiency and that diversion colitis may be superimposed by a second insult, such as a low-grade pathogen.