To explore the effect of hMLHland hMSH2 in carcinogenesis and progression of colorectal carcinoma,and their influences on proliferation in colorectal adenoma and malignant change in adenomas. Methods we investigated the expression of mismatch repair (MMR) gene hMLH1, hMSH2 and proliferation cell nuclear antigen (PCNA)by immunohistochemistry in 63 cases colorectal adenomas,20cases malignant change in adenomas and 20cases colorectal carcinomas (CRCs).Results The positive rate of hMLH1 and hMSH2 in colorectal adenomas,malignant change in adenomas and CRCs were significantly lower than that in normal mucosa. With increasing dysplasia in adenomas, the expression rate of hMLH1 and hMSH2 protein decreased gradually;, The PCNA label index of tumors with overexpreasion hMSH2 were significantly higher than that of tumors with negative hMSH2 in colorectal malignant change type in adenomas and CRCs PCNA lable index of tumors with overexpression hMSH2 were significant higher than that of tumors with negative hMSH2 in adenomas with grade Ⅱ, Ⅲ dysplasia. The changes of PCNA label index in colorectal adenomas, malignant change type in adenomas and CRCs were not association with the expression of hMLH1. Conclusion It indicated that the MMR genes mutation and abnormal function of DNA mismatch repair maybe participate in carcinogenesis of CRCs.It is might be an early event in carcinogenesis of CRCs. It suggested that the reduction of proliferating activities in colorectal neoplasms might be related to mutation and defect of MMR gene.