Nuclear immunoreactivity for beta-catenin is a useful adjunct for diagnosis of adult desmoid-type fibromatoses, many of which exhibit mutations within the APC/beta-catenin (Wnt) pathway. Pediatric fibromatoses represent a heterogeneous group of lesions that are diagnostically challenging, especially on biopsy. We studied beta-catenin expression in a variety of pediatric fibroblastic and myofibroblastic lesions. Immunohistochemical nuclear expression of beta-catenin was assessed in 100 tumors. High-level expression of beta-catenin was found in 42% of usual-type or deep fibromatoses (21 of 50). Such expression was not seen in any of the other lesions, including fibrous hamartoma of infancy (0 of 18), juvenile hyaline fibromatosis (0 of 7), infantile digital fibromatosis (0 of 6), myofibromatosis (0 of 5), lipofibromatosis (0 of 4), calcifying aponeurotic fibroma (0 of 3), palmar-plantar fibromatosis (0 of 2), fibromatosis colli (0 of 1), or torticollis (0 of 1). High-level beta-catenin staining is seen in deep "adult-type" fibromatoses occurring in children, although to a lesser frequency than in adult fibromatoses. This indicates that a subset of deep fibromatoses in childhood shares similar mechanisms of tumorigenesis with those in adults. beta-catenin is not expressed in other common pediatric fibroblastic and myofibroblastic lesions, and the Wnt pathway does not appear to play a role in their pathogenesis.