Aims: Desmoid-type fibromatosis (DF) is a rare benign myofibroblastic neoplasm of the connective tissue that is unable to metastasize but is associated with a high local recurrence rate. Nuclear b-catenin is the most commonly used histological marker of DF; however, clinical and biological predictive markers guiding the treatment and follow-up of DF are still lacking. Normally, b-catenin is regulated by the cytoplasmic multiprotein complex of adenomatous polyposis coli (APC), axin, casein kinase 1a (CK1a), and glycogen synthase kinase 3b (GSK-3b); this phosphorylates and degrades b-catenin, which would otherwise translocate to the nucleus. The aim of this study was to analyse the expression and localization of the b-catenin–protein complex of the Wnt pathway in cells isolated from DF patients. Methods and results: We isolated cells from biopsies of DF patients, and demonstrated, by immunofluorescence and immunoblot analyses, that it is almost exclusively nuclear GSK-3b that colocalizes and interacts with b-catenin. The nuclear translocation of b-catenin and GSK-3b is not correlated with CTNNB1 mutations. In DF samples, the multiprotein complex is disrupted, as the cytoplasmic localization of APC and axin makes interaction with the nuclear b-catenin and GSK-3b impossible. Conclusions: Our data suggest that GSK-3b is an additional DF marker with an important role in the aetiopathogenesis of this entity.