In a 15-year surveillance program composed of 72 patients with total ulcerative colitis, 12 patients developed definite dysplasia. At endoscopy, low-grade dysplasia was detected in seven patients, high-grade in four, and a carcinoma (Dukes' stage A at operation) in one. One of the patients with high-grade dysplasia and macroscopical lesions at colonoscopy had a carcinoma (Dukes' A) detected at operation. A sequential development of dysplasia was found in seven patients. The cumulative risk of developing at least low-grade dysplasia was 14% after 25 years of disease duration. Using flow cytometric analyses, abnormal, aneuploid DNA content was detected in biopsies of 12 of 59 patients (20.3%); this correlated significantly with low-grade and high-grade dysplasia. Aneuploidy preceded dysplasia in two patients and was also detected in two dysplasia-free patients. The long-term use of colonoscopic surveillance in ulcerative colitis is a reliable way to select patients, in whom dysplasia is developing, for prophylactic surgery. Additionally, flow cytometric DNA analyses may help in the selection. The risk of missing a carcinoma until it becomes incurable appears to be low.