We comparatively assessed 41 mucinous colorectal carcinomas (MUCs) and 620 non-MUC (well-, moderately, and poorly differentiated adenocarcinoma) cases for clinicopathologic findings; and 41 MUCs and 115 randomly selected non-MUCs also were studied for the following: (1) apoptotic activity and Ki-67 immunoreactivity; (2) immunohistochemical expression of p21(WAF1/CIP1), p27Kip1, p53, and bcl-2; and (3) c-Ki-ras mutations. The rates for lymph node involvement and peritoneal dissemination were higher in MUCs than in non-MUCs. Multivariate analysis showed MUCs to have a worse prognosis than well-differentiated adenocarcinomas. The Ki-67 labeling for MUCs was significantly lower than that for non-MUCs, whereas the apoptotic index was significantly higher than for the well-differentiated type. The labeling for p21(WAF1/CIP1) and p27Kip1 was lower in MUCs (2.7% and 35.3%, respectively) than in well-differentiated adenocarcinomas (4.2% and 48.6%, respectively). MUCs can be considered a different tumor from the well-differentiated type, with a poor prognosis owing to frequent lymph node metastasis and peritoneal dissemination, and characterized by high apoptotic and low proliferative activities associated with low p21(WAF1/CIP1) and p27Kip1 expression.