Severe epithelial dysplasia in multiple colonscopic biopsy specimens may held identify those patients with chronic ulcerating colitis (CUC) at highest risk for developing colonic cancer. This retrospective analysis of 75 patients with longstanding CUC studied by colonoscopy and biopsy disclosed 11 patients with cancer. Four of these cancers were detected by surveillance colonoscopy and seven were detected by diagnostic colonoscopy. There was a high correlation between the presence of dysplasia and cancer in the patients. Cancer was found in five of seven patients who underwent operation after colonoscopic biopsies showed severe dysplasia. No cancers were found at operation in 15 patients whose colonoscopic biopsy specimens were free of dysplasia. Dysplasia can be consistently identified. A reviewing pathologist agreed with the initial biopsy diagnosis in 59 of 64 cases (92%) and with the colectomy diagnosis in 18 of 21 (86%). Colonscopy with multiple biopsy specimens is a valuable surveillance technic. Patients without dysplasia on biopsy should have continued surveillance.