In a prospective study carried out between 1977 and 1991, 131 patients with ulcerative colitis from an area of 65,000 inhabitants were followed by clinical visits and regular colonoscopy with biopsy. At the end of the study 58 patients had had total colitis for more than 10 years, 38 had left the programme (16 after radical operations, nine because of death--one from colitis, one from carcinoma--and 13 for other reasons) and 632 colonoscopies had been performed. Colorectal carcinoma was diagnosed in four patients, of whom two were included in the programme with a diagnosis of cancer. Dysplasia was diagnosed in 24 patients, other than those with malignancy; in four this was of high grade. Carcinoma and dysplasia occurred mainly in the left colon and in patients with total colitis. The surveillance programme was resource consuming and the cost-benefit must be questioned.