It has been reported that wild-type APC protein forms a complex with beta-Catenin and GSK3beta, inducing degradation of beta-Catenin in normal cells. Both beta-Catenin and APC gene mutations have recently been shown to activate the same signaling pathway. Frequent mutations of beta-Catenin in hereditary nonpolyposis colorectal carcinomas have also been reported. It was, however, controversial whether the mutation of the beta-Catenin gene was frequent in nonfamilial colorectal carcinomas with high-frequency microsatellite instability (MSI-H). We analyzed the mutations of the APC and beta-Catenin genes in 56 nonfamilial colorectal carcinomas stratified according to the presence or absence of microsatellite instability (MSI). APC mutations were identified in 11 of 22 (50%) cases of MSI-H and 14 of 34 (41%) cases of microsatellite-stable (MSS)/low-frequency microsatellite instability (MSI-L). In contrast, the frequency of beta-Catenin mutations was significantly higher in MSI-H (6/22; 27%) than in MSS/MSI-L (1/34; 3%) (P = 0.01). beta-Catenin mutations were not detected in carcinomas with APC mutation. APC mutation occurred irrespective of MSI status. beta-Catenin mutation, however, occurred frequently in MSI-H carcinomas. Our data suggest that activation of the beta-Catenin-Tcf signaling pathway, through either beta-Catenin or APC mutation, frequently contributes to MSI-H nonfamilial colorectal carcinomas (17/22; 77%).