The authors studied eight colectomy and eight biopsy specimens from 12 patients with cystic fibrosis who had developed fibrosing colonopathy, a complication observed in patients receiving high-strength enzyme replacement. The colectomies originated from five male and three female patients ranging in age from 18 months to 6 years. Five individuals had localized strictures of the right colon and three had stenosing fibrosis of the entire colon. The affected colon had a cobblestone appearance, submucosal fibrosis, thickening of the muscularis propria and chronic mucosal inflammation in all patients, with active cryptitis in four. Moderate to severe infiltration by eosinophils, with increase in the number of mast cells, and widespread interruption of the muscularis mucosa were present in every case. Four colectomies were preceded by endoscopic biopsies; four patients who have not undergone surgery also underwent biopsy. All the biopsies showed evidence of active or chronic inflammation, and all had increased mucosal eosinophils. Prolonged colonic mucosal contact with either the enzymes and/or the enteric coating itself may lead to mucosal colonic ulceration and inflammation. Topical allergy may then promote the stenosing fibroplasia.