Using cytofluorometric single-cell DNA analysis, the pattern of nuclear DNA content was determined in preoperative biopsies from 166 unselected primary adenocarcinomas of the rectum or rectosigmoid. The pattern was aneuploid (AN) in 125 (75%) of the tumours, was aneuploid (AN) in 125 (75%) of the tumours, near-diploid with aneuploid cells (ND-anc) in 26 (16%) and near-diploid (ND) in 15 (9%). The DNA content was not related to tumour stage or degree of differentiation. The clinical course tended to be more favourable in patients with ND tumour than in those with AN or ND-anc pattern. The nuclear DNA content in rectal carcinoma thus can be estimated in preoperative biopsies, and the pattern may serve as a prognostic parameter.