Juvenile nasopharyngeal angiofibromas (JNAs) are locally aggressive vascular tumors occurring predominantly in adolescent males. The pathogenesis of JNAs is unknown. Recently, JNAs have been reported to occur at increased frequency among patients with familial adenomatous polyposis, suggesting that alterations of the adenomatous polyposis coli (APC)/beta-catenin pathway might also be involved in the pathogenesis of sporadic JNAs. We analyzed somatic beta-catenin and APC gene mutations in 16 sporadic JNAs from nonfamilial adenomatous polyposis patients using immunohistochemistry for beta-catenin, and direct DNA sequencing for exon 3 of the beta-catenin gene and the mutation cluster region of the APC gene. Nuclear accumulation of beta-catenin was diffusely present in the stromal cells but not in the endothelial cells of all 16 JNAs. Activating beta-catenin gene mutations were present in 75% (12 of 16) of JNAs. Six JNA patients also had recurrent tumors after surgery, and in all cases the beta-catenin gene status of the recurrent JNA was identical to the initial tumor. No mutations in the mutation cluster region of the APC gene were detected in the four JNAs without beta-catenin mutations. The high frequency of beta-catenin mutations in sporadic JNAs and the presence of identical beta-catenin gene mutations in recurrent tumors indicates that activating beta-catenin gene mutations are important in the pathogenesis of JNAs. The immunohistochemical localization of beta-catenin only to the nuclei of stromal cells further suggests that the stromal cells, rather than endothelial cells, are the neoplastic cells of JNAs.