The determination of proliferative activity in the colorectal mucosa has been used for different purposes as the estimation of cancer risk and the assessment of disease activity in ulcerative colitis. But the prognostic significance of proliferative activity in colorectal carcinomas remains controversial. To investigate the prognostic significance of proliferative activity in colorectal carcinomas, the author estimated the proliferative activity immunohisto chemically using the monoclonal antibody PCNA and compared with clinicopathological data in 62 colorectal carcinomas. The results were as follows: 1) The reactivity of PCNA was more pronounced at the infiltrative margins of the tumors and tumor cells within the vascular or lymphatic channels. 2) The mean PCNA index of colorectal carcinomas was 40.5?0.4%. PCNA indices had positive correlations with lymph node invasion(p<0.05), liver, metastasis(p<0.05), Dukes' stage(p<0.01) and TNM classification(p<0.01), and didn't correlated with location of tumor, size of tumor, histological type and lymphtic or vascular invasion. 3) The patients with high PCNA index(more than 45%) represented higher recurrence or metastasis rate(37.5%) than those with low PCNA index (less than 45%)(19.3%) in Dukes B or C colorectal carcinomas during the follow-up periods, but not significant statistically. These results suggested that the reactivity of PCNA may be a useful prognostic factors in colorectal carcinomas.