During a 13-year period, 213 patients with ulcerative colitis who had no clinical or endoscopic evidence of colonic carcinoma were enrolled in a biopsy surveillance program for dysplasia and carcinoma. The aims of the study were to determine whether such a program could decrease the cancer risk in this group of patients, to determine whether patients with a low risk of carcinoma could be identified, thus enabling them to retain their colon, and to accomplish these goals with a reasonable expenditure of resources. Eighteen patients had dysplasia detected in the initial biopsy specimens; 15 of these patients underwent colectomy, and 7 had unsuspected carcinoma (1 Dukes' stage A, 2 stage B, and 4 stage C). Eleven patients had dysplasia detected during follow-up; 7 of these patients had colectomy, and only 1 patient had carcinoma (Dukes' B). Dysplasia developed in 5 of 20 patients with indefinite changes on initial biopsy samples; 3 of these patients underwent colectomy, and 1 patient had carcinoma (Dukes' B). There was no difference in the prevalence of dysplasia between patients with left-sided disease and patients with extensive disease. With the exception of 2 patients with inadequate surveillance, there has been no clinical evidence of carcinoma in any of the 148 patients whose biopsy results remained negative throughout the study; carcinoma has not developed in any of 175 patients without dysplasia on initial biopsy sample. All 4 patients who died of carcinoma had high-grade dysplasia in their initial colonoscopic biopsy samples. It is concluded that a biopsy surveillance program can be an effective aid in helping control the risk of carcinoma in patients with long-standing ulcerative colitis, that the short-term risk of carcinoma for patients with negative biopsy results is low and colectomy for risk of carcinoma can be deferred in this group, and that patients with extensive and left-sided disease share the same risk of the development of dysplasia.