Proliferating cell nuclear antigen(PCNA) is an auxiliary protein of DNA polymerase 8 and is considered to correlate with the proliferative state of cells. If such a biologic marker in colorectal cancer tissue correiates with recurrence and poor survival, it would offer a rationale for planning aggressive adjuvant therapy. Authors investigated the prognostic significance of proliferative activity in cancer cells in patients with colorectal carcinoma using PCNA immunohistochemical analysis. An anti-PCNA monoclonal antibody (PC 10) was used to measure proliferation indices in neoplastic colorectal tissues of 61 patients with the different clinical outcomes. The correlations of PCNA labeling index (PCNA LI) with various clinicopathologic factors, recurrence and prognosis were studied. The results obtained were summarized as follows; 1) The PCNA LI become higher as the histologic differentiation decreased and the Dukes stage increased, both of which were statistically significant (p=0.0133 and 0.0001, respectively). 2) The rate of disease recurrence after curative resection (53 cases) was significantly higher in patients with high PCNA LI (PCNA LI>=50) as compared with those with low PCNA LI (PCNA LI<50) (44.4% vs. 14.3%, p= 0.022). 3) In patients with high PCNA LI, prognosis (2-year survival) was significantly poorer than in those with a low index (31.8% vs. 58.6%, p<0.05). As a result of this study, PCNA labeling index as determined by PCNA immuno- histochemical analysis is suggested to be correlated well with the histologic differentiation and tumor stage and to be an effective predictor in colorectal cancer. The PCNA analysis for biologic heterogeneity of patients with colorectal cancer may be useful in the prognostic grouping of patients and planning prophylactic therapy.