Abstract: Objective :To evaluate the significance of the lack of expression of mismatch repairgenes (MMR) in screening for hereditary nonpolyposis colorectal cancer(HNPCC). Methods :One hun-dred sixty-six patients with suspected HNPCC that were younger than 40 years of age were assessed forDNA mismatch repair deficiency by immunohistochemical detection of the hMLH1 and hMSH2 geneproducts. Results :Of the 166 patients, 25 (15.53%) showed no detectable expression of hMLH1, 136(84.47%) showed normal expression of hMLH1, 35(21.08%) showed no detectable expression of hMSH2,and 126 (75.9%) showed normal expression of hMSH2. Loss of expression of at least one of these MMRproteins was found in 80 of the 166 (48.19%). The rate of negative hMLH1 protein expression in tumorsin the right colon, left colon and rectum was 26.67%(12/45), 5.13% (2/39), and 14.29%(11/77), respec-tively, and the rate of negative hMSH2 protein expression in tumors in the right colon, left colon andrectum was 33.33%(15/45), 15.38%(6/39) and 18.18%(4/77), respectively. Conclusion :Our results sug-gest that defects in the DNA mismatch repair system are frequent in young Chinese patients with col-orectal cancer. Imunohistochemical detection is useful for identifying HNPCC among young patientswith colorectal cancer.