A prospective study of prognostic factors has been carried out in a group of 123 consecutive patients with colorectal cancer. The fate of all patients is known at 3 years after operation. Clinical and pathological data were recorded at the time of presentation and operation, and the patients have been subject to regular postoperative review. DNA ploidy status was determined by flow cytometry. In all, 39 (33 per cent) patients had DNA diploid tumours and 80 (67 per cent) patients had DNA aneuploid tumours. In four cases, tumour material was not obtained. The patients with DNA aneuploid tumours had a worse prognosis than those with DNA diploid tumours, but this was only seen in those patients classified as Dukes' B. In a Cox's regression analysis, the surgeon's assessment of operability was the strongest predictor of survival, followed by the pathological classification and the patient's age. After these factors had been considered, the DNA ploidy status conferred no independent survival value.