The cancer risk among 229 patients with extensive ulcerative colitis observed during January 1, 1966 to February 29, 1976 is correlated with the length of history and the histological findings in rectal and colonic biopsies. Five patients are known to have developed carcinoma. No carcinoma was observed in 578 patient years of follow-up within 10 years of onset of the colitis, but the risk in the second decade was approximately 1 in 200 patient years and in the third, 1 in 60 patient years. Severe epithelial dysplasia was rare and found in 32 biopsies from 13 patients. No carcinoma has occurred during the period of follow-up in patients without dysplasia. It has not been possible to follow the development of dysplasia in sequential biopsies. Seven patients with consistent severe dysplasia on biopsy have been treated surgically; carcinoma confined to the bowel wall (Dukes' A) was found in 4. A scheme of management for patients with extensive colitis, including regular rectal and colonic biopsies, is proposed. Our results suggest that such a program will isolate a small group of patients who require surgical treatment for established precancerous change or carcinoma with a high likelihood of cure.