DNA ploidy and cell cycle phases were evaluated by flow cytometry in colonic biopsy specimens from 107 patients with ulcerative colitis in order to analyse the prevalence of DNA aneuploidy as an indicator of numerical chromosomal aberrations and the cell proliferation in all forms of ulcerative colitis. Whereas G2/M-phase fractions in ulcerative colitis and in controls were comparable (2.7 +/- 1.1% vs. 2.8 +/- 1.1%), S-phase fractions in ulcerative colitis exceeded those of controls (7.5 +/- 3.2% vs. 6.5 +/- 2.3%; p < 0.01). In 28 control patients, only diploid DNA histograms existed. Single or multiple aneuploid stem lines were detected in 10 patients with ulcerative colitis (9.3%). Aneuploidy was nearly exclusively associated with pancolitis. Dysplasia was present in 13 patients (indefinite: 8; low-grade: 5), of whom 5 patients also showed DNA aneuploidy. 5 patients with non-dysplastic mucosa exhibited DNA aneuploidy. Because dysplasia and DNA aneuploidy can be discordant and might therefore identify different subgroups at risk, flow cytometry might play a role as a valuable complement to histological examination in surveillance programs of ulcerative colitis.