To evaluate the efficacy of surveillance colonoscopy with biopsy for the detection of high-grade dysplasia (HGD) or colonic carcinoma in patients with chronic ulcerative colitis, we undertook a retrospective review of 248 patients who underwent 370 examinations (mean duration of disease 12 yr). High-grade dysplasia or carcinoma was found in 24 examinations in 16 patients, with a mean duration of disease of 16 yr. There were 15 patients with HGD. Nine patients had HGD alone, 6 had HGD and carcinoma, and 1 had carcinoma without HGD. The overall incidence of HGD was 6%. Dysplasia-associated lesions or mass were the most consistent indicators of carcinoma, the combination being present in four instances. Of the 7 patients with cancer, 6 were recognized by colonoscopy, and 1 patient with negative visual endoscopic findings was discovered using surveillance biopsies. The conclusions of this study are that dysplasia is a reliable histopathologic marker and correlates with the presence of cancer in chronic ulcerative colitis; the absence of dysplasia correlates with the absence of cancer. The presence of dysplasia-associated lesions or mass with HGD is the strongest indication for operation. This study supports the use of surveillance colonoscopy in managing high-risk ulcerative colitis patients.