The purpose of this study was to assess the usefulness of nuclear DNA analysis in early cancer detection in patients with ulcerative colitis. The results of flow cytometric nuclear DNA anaiysis of tissue specimens from patients with ulcerative colitis, colonic adenoma, and colorectal carcinoma were compared. DNA aneuploidy was observed in 53.1 % of patients with ulcerative colitis and 16.7 % of patients with colonic adenoma. The degree of atypism of the tumor cells was related to the incidence of DNA aneuploidy. DNA aneuploidy was noted in 48.6 % of the patient with ulcerative colitis and coexistent colorectal carcinoma. The degree of dysplasia of the tumor cells was also correlated with the incidence of DNA aneuploidy. DNA aneuploidy was found in 14.3 % of specimens of mucosal tissue that were without cancerous or dysplastic cells. DNA aneuploidy was found in 8.5 % of specimens of mucosal tissue collected from patients with ulcerative colitis alone. Our findings showed that abnormalities in DNA content was closely correlated with histological atypism. In addition, our findings suggested that DNA aneuploidy may precede dysplastic changes in some cases of ulcerative colitis. We concluded that DNA analysis using flow cytometry is a useful method for identification of patients with ulcerative colitis at high risk for the development of cancer.