A prospective study of patients who had undergone fecal diversion was performed in order to determine the incidence of and to characterize better the condition known as "diversion colitis." A total of 53 patients were studied. All patients underwent endoscopic evaluation of the diverted large bowel. Evidence of colitis was found in 48 patients (91%). The colitis was mild in 52%, moderate in 44%, and severe in only 4%. Endoscopic findings included: contact irritation or bleeding, erythema, and mucosal nodularity. The proximal, "in continuity," colon was examined in 86% of patients with colostomies; none were found to have colitis. Biopsies were taken of the diverted segment in 94% of patients with colitis and from the "in continuity" colon in 78% of patients with colostomies. Similar histologic findings were noted on these biopsies and included: mild chronic inflammation, lymphoid nodules, and crypt architectural changes. With the exception of lymphoid nodules, which were seen more frequently in the inflamed diverted colon (P = 0.035), there was no significant difference in the incidence of the various histologic changes when the biopsies from the diverted and "in continuity" large bowel were compared. Rectal washings and stool samples were sent for bacterial cultures, ova, and parasite analysis, and C. Diff. toxin titers in the majority of patients; all but 1 were negative. Symptoms relating to the diverted bowel were elicited in only 3 patients (6%). Stomal closure was carried in 70% of patients. Postclosure endoscopy in 21 patients revealed full resolution of the colitis in all. Diversion colitis occurs in almost all diverted patients. It uniformly resolves following stomal closure.(ABSTRACT TRUNCATED AT 250 WORDS)