This study sought to examine the relationship between MUC1 expression at the deepest invasive portion, invasive/metastatic potential, and prognosis of colorectal cancer in relation to cellular proliferation, MUC1 expression was detected immunohistochemically using KL-6 antibody (anti-MUC1 monoclonal antibody) in 100 surgically resected specimens of advanced colorectal cancer. Distinct staining of the luminal surfaces, defined as positive immunoreactive (IR)-MUC1 expression, was Seen in more than 30% of the tumor cells at the deepest invasive portion. The proliferating cell nuclear antigen labeling index (PCNA-LI) was also examined in the same areas. IR-MUC1 expression was detected in 71 (71%) of 100 lesions. Lesions with lymphatic or venous invasion showed a significantly higher incidence of IR-MUC1 expression than those without lymphatic or venous invasion (80 vs. 42% and 82 vs. 61%, respectively). Lesions with lymph node metastasis showed a significantly higher incidence of IR-MUC1 expression than those without lymph node metastasis (88 vs. 53%). Lesions with liver metastasis showed a significantly higher incidence of IR-MUC1 expression than those without liver metastasis (92 vs. 59%), Dukes' stage was also significantly correlated with IR-MUC1 expression. The incidence of IR-MUC1 expression did not significantly differ with regard to histologic subclassification and depth of invasion. There was no significant correlation between IR-MUC1 expression and the PCNA-LI. IR-MUC1 expression at the deepest invasive portion revealed a significant correlation with prognosis; furthermore, in patients with better differentiated lesions, in those with lesions confined to muscularis propria or subserosa (subadventitial) invasion, in those with Dukes' B and C, or in those undergoing curative resection, IR-MUC1 expression significantly correlated with prognosis. Patients with high PCNA-LI lesions showed a significantly poorer prognosis than those with low PCNA-LI lesions. Only in patients undergoing curative resection, patients with IR-MUC1-positive and high PCNA-LI lesions showed a significantly poorer prognosis than those with IR-MUC1-negative and low PCNA-LI lesions. The significant risk factors in the order of poorer prognosis in patients undergoing curative resection by the multivariate analysis were the histologic grade (moderately-poorly, poorly or mucinous adenocarcinomas), IR-MUC1 expression, and lymph node metastasis, These results indicate that IR-MUC 1 expression is an important predictor of the metastatic potential and the prognosis of colorectal cancer, independent of histologic grade, depth of invasion or cellular proliferative activity. Combined analysis of IR-MUC1 and histologic grade, and combined expression of IR-MUC1 and PCNA at the deepest invasive portion are especially useful in predicting colorectal cancer prognosis.