The risk of colon cancer in patients with ulcerative colitis is related to the duration and extent of disease. Prior reports have suggested that patients with onset of disease in childhood have a high risk of cancer. These risk factors were analyzed in 99 patients in a surveillance program of annual colonoscopy to detect mucosal dysplasia. All patients had pancolitis for at least eight years. The mean age at symptom onset was 23.2 years and the mean duration of disease at entry was 17 years. An average of 4.2 tests/patient were performed, and 91% were completely followed through 1985. Cancer risk was expressed as the hazard rate or the annual probability that a patient free of cancer would develop cancer after survival to a given time period. The hazard rate for high-grade dysplasia or cancer in patients with pancolitis measured from symptom onset was 2.5% at 20 years, 4% at 25 years, 7% at 30 years, 13% at 35 years, and 20% at 40 years. Sex was not a significant predictor of cancer, but older age at symptom onset was a predictor of dysplasia and cancer. From these data, the annual hazard rate of developing high-grade dysplasia or cancer can be estimated in patients with pancolitis based on an individual's age at symptom onset and duration of disease.