DNA flow cytometry was performed on paraffin-embedded tissue blocks made from 230 surgically resected colorectal cancers, 109 (47.4%) of which were diploid tumors, and 121 (52.6%) aneuploid tumors. The DNA index (DI) was calculated as the ratio of the G0/G1 channel number of tumor cells to the G0/G1 channel number of stromal cells. There was no significant difference in survival between patients with diploid tumors and those with aneuploid tumors (P = 0.322), although the survival rate was significantly lower in patients with a high DI (> or = 1.5) than in those with a low DI (< 1.5) (P = 0.004). A multivariate analysis of prognostic factors using Cox's proportional hazard model showed that Dukes' staging was the strongest predictor of survival, followed by the DI of tumor cells, then histological differentiation. In conclusion, it is suggested that the DI of tumor cells is instructive for predicting the survival of patients with colorectal cancer.