The DNA content of 181 colorectal cancers was investigated by flow cytometry on paraffin-embedded specimen. The relationship of flow cytometric DNA patterns of colorectal cancer to Dukes's stage, histological type and grade, tumor size and patient's prognosis were analysed. The DNA aneuploid carcinomas were found in 53.04% of the patients (hypodiploid 22.10%, hyperdiploid 20.99%, polyploid 9.94%). Diploidy was found in 46.96% of the patients with colorectal cancer. The proportions of aneuploidy were significantly lower in the patients with Dukes' stage A (25.00%) and B (47.14%) than those with Dukes' stage C (64.71%) and D (68.42%). Aneuploid frequency was higher in the patients with poorly differentiated adenocarcinoma (84.62%), mucinous adenocarcinoma (62.50%) and tubular adenocarcinoma (52.75%) than those with villous adenocarcinoma (22.86%). The proportion of aneuploidy was significantly higher in the patients with poorly differentiated tumor (71.79%) than in those with moderately (50.00%) and well differentiated tumors (44.83%). Five-year survival rate of the patients with DNA aneuploid tumors was 33.80% compared with 66.67% of those with diploid tumors. Analysis of Cox regression revealed that DNA ploidy and Dukes' stage significantly influenced the patient's prognosis. It is suggested that DNA ploidy might be an important prognostic factor in colorectal cancer.