Surveillance has become an established concept to find dysplasia as a premalignant lesion in ulcerative colitis. Since the advent of colonoscopy and the access of the entire colon to biopsy, utilization of the rigid sigmoidoscope and rectal biopsies has been markedly reduced. We have reviewed 940 rectal biopsies obtained from 360 patients with ulcerative colitis of various extent and duration to determine the yield of dysplasia and assess the value of this procedure. The rectal biopsies had been performed for a variety of reasons in the course of clinical management of ulcerative colitis in addition to surveillance. Dysplasia was found in 10.8% of the patients—higher than has been demonstrated by colonoscopy. In four of the 39 patients with dysplasia a carcinoma coexisted (2) or developed subsequently (2). In no instance did dysplasia serve to warrant the colectomy that disclosed the carcinoma, but earlier discovery of the dysplasia or more intense surveillance once it had been found might have led to earlier discovery of the carcinoma. The yield of dysplasia was greater in those with onset of ulcerative colitis in childhood. In more than half, including 71% of patients with onset in childhood, the dysplasia was found before they had the disease for 9 years. Surveillance, therefore, should begin earlier in young patients with ulcerative colitis. Sigmoidoscopic biopsies do not substitute for colonoscopic biopsies in a surveillance program. Nevertheless, the ease of obtaining rectal biopsies, the relatively large yield of dysplasia, and the need for frequent sigmoidoscopy in the clinical management of ulcerative colitis all warrant a high priority role for rectal biopsies to supplement that of colonoscopic biopsies.