A total of 63 patients with colorectal cancer were studied for proliferative activity by an immunohistochemical technique using a monoclonal antibody against DNA polymerase alpha. The DNA polymerase alpha positive cell rates ranged from 24.0% to 74.6%. There was a correlation between the DNA polymerase alpha positive cell rates of biopsies and resected specimens. There was no significant correlation between DNA polymerase alpha positive cell rates and histological type, tumour size, invasion of bowel wall, lymphatic invasion, venous invasion, lymph node metastasis or peritoneal metastasis. Tumours with a high growth fraction (a DNA polymerase alpha positive cell rate greater than or equal to 42%) were more frequently associated with liver metastasis than those with a low growth fraction (a DNA polymerase alpha positive cell rate less than 42%). Patients with high growth fraction tumours had significantly poorer prognoses than those with low growth fraction tumours. The results of multivariate analysis using the proportional hazard model of Cox indicated that the DNA polymerase alpha positive cell rates, liver metastasis, and peritoneal metastasis were independent prognostic factors. The results indicate that the DNA polymerase alpha positive cell rate may be a useful prognostic marker of colorectal cancers.