The cellular DNA content was measured from paraffin-embedded material in 134 colorectal cancers from patients in whom the outcome was known. Seventy-two (55 per cent) were found to contain cells with abnormal DNA (DNA aneuploid). The presence of such a population of cells was not related to pathological stage or histological grade. However, only 14 (19 per cent) patients with DNA aneuploid tumours survived 5 years compared with 27 (43 per cent) of patients with diploid tumours (chi 2 = 8.0, P = 0.005). Stepwise logistic analysis showed cellular DNA content to be an important prognostic factor in colorectal cancer, independent of pathological stage and histological grade.