Wnt signalling pathway is known to have a critical role in carcinogenesis and in epithelial-to-mesenchymal transition. Upon Wnt activation, β-catenin is translocated from the membrane to the cytoplasm and nucleus, where it interacts with transcriptional activators. It has been suggested that various spindle cell lesions of the breast may harbour Wnt pathway activation. Given that β-catenin nuclear localization constitutes a good surrogate marker of Wnt canonical pathway activation, we have investigated the distribution of β-catenin in spindle cell lesions of the breast and whether it could be employed in the differential diagnosis of these lesions. A total of 52 metaplastic breast carcinomas, eight fibromatoses and 23 phyllodes tumours were retrieved from our institutions' archives. We performed immunohistochemistry using two anti-β-catenin antibodies. In all, three fibromatoses and 21 metaplastic breast carcinomas were subjected to CTNNB1 (β-catenin encoding gene) mutation analysis by direct gene sequencing. A good correlation between the two antibodies was observed (Spearman's r>0.82, P<0.001). All fibromatoses and 23% of metaplastic breast carcinomas expressed nuclear β-catenin. In fibromatosis, β-catenin was more often diffusely expressed, whereas in metaplastic breast carcinomas, expression was more frequently focal. Membranous β-catenin expression was significantly lower in spindle cell carcinomas than in other subtypes of metaplastic breast carcinomas. In phyllodes tumours, stromal cells of benign and malignant subtypes displayed nuclear β-catenin expression in 94 and 57% of cases, respectively. No CTNNB1 mutation was identified in any of the 21 metaplastic carcinomas analysed, whereas the mutations 45S>S/P and 41T>T/A were found in samples of fibromatosis. In conclusion, β-catenin nuclear expression is a common feature in fibromatoses and in the stromal component of phyllodes tumours, but may also be observed in metaplastic breast carcinomas. β-catenin nuclear expression should not be used as a single marker to differentiate fibromatosis from other spindle cell tumours of the breast.