BRCA1 is a nuclear phosphoprotein that participates in the regulation of the cell cycle. The role of the BRCA1 gene in canine mammary tissue and mammary tumours has not been studied. The present study examined immunohistochemically the expression and intracellular distribution of BRCA1 protein in two normal, seven dysplastic and 44 neoplastic canine mammary glands and its relationship with clinical and pathological variables and other prognostic parameters. Strong nuclear immunolabelling of BRCA1 protein was observed in the epithelial cells of the normal mammary glands and mammary dysplasias. The majority of benign tumours, and more especially of malignant tumours, showed a significant reduction in the nuclear expression of BRCA1 protein and an increase in cytoplasmic expression. Loss of BRCA1 expression was associated with high proliferation marker Ki-67 and ER-alpha negative tumours. The reduction and aberrant distribution of BRCA1 in canine mammary tumours were significantly associated with malignant characteristics. The results may indicate that BRCA1 has a role in the malignant behaviour of these tumours.