The prevalence of deoxyribonucleic acid (DNA) aneuploidy in 297 samples from 38 patients with ulcerative colitis of varying duration was investigated by flow cytometry. In 12 patients colitis was complicated by the development of colorectal carcinoma: one had three synchronous carcinomas. Only four of 14 carcinomas were DNA aneuploid. Deoxyribonucleic acid aneuploidy occurred focally in the colorectal mucosa in the presence and absence of carcinoma: rates of aneuploidy (67% in cancer patients and 42% in non-cancer patients), were not significantly different (chi 2 = 1.0962, p = 0.295). A higher rate of DNA aneuploidy was found in dysplastic tissues (21%) compared with non-dysplastic tissues (15%), but again these differences did not reach statistical significance (chi 2 = 1.0747, p = 0.299). Deoxyribonucleic acid aneuploidy and dysplastic change occurred more often with increasing duration of ulcerative colitis (p less than 0.001, p less than 0.005 respectively). We conclude that flow cytometric analysis of cellular DNA content should not replace present morphological methods of assessment of premalignancy in ulcerative colitis, but may be a useful adjunct in the identification of abnormal mucosa.