The aim of this study was to determine the correlation between microsatellite instability (MSI) and young age in patients with advanced colorectal adenomas. We retrospectively analyzed young patients (≤40years of age) with advanced adenomas (n=84) between January 1996 and December 2006. We randomly selected the control group as patients≥50years of age with advanced adenomas (n=84) during the same time period. Of these patients, the MSI test and MLH1 immunohistochemistry were performed in the available tissue samples from patients with advanced adenomas. The number of patients who had the two tests was 52 in the young group and 49 in the old group. The monomorphic nature of the BAT26 panel for MSI analysis was used without comparison of normal tissue. MSI was detected in three young patients (n=52) and none of the old patients (n=49). There was no statistical difference between the two groups (P=0.243). All three young patients with MSI had a strong family history of colorectal cancer. MSI analysis was not a useful method of screening for HNPCC in young patients with advanced colorectal adenoma, at least in cases without a family history of colorectal cancer.