Sixty patients with ulcerative colitis of more than nine years' duration and 13 patients with a shorter disease duration were examined over a three-year period with flow cytometric DNA analysis and conventional histologic investigation of biopsy specimens obtained by colonoscopy. The patient series included all patients with ulcerative colitis from a colonic cancer surveillance program in a defined area. There were five patients-one who developed colonic cancer, one with high-grade dysplasia, and three with low-grade dysplasia. None of these showed aneuploidy (abnormal DNA stem lines). There were six patients with aneuploidy in one to six locations in the colorectum. On consecutive examinations, there was a high reproducibility of the abnormal DNA pattern for ploidy levels and location in the bowel. In these patients, there was no obvious relationship with definite dysplasia. It is concluded that flow cytometric DNA analysis in ulcerative colitis may become a valuable complement to evaluation of dysplasia in cancer surveillance, but assessment of the cancer risk in individual patients with aneuploidy needs further investigation.