Abstract Expression of the luminal cytokeratin CK19 was examined in 102 canine mammary carcinomas by immunohistochemical analysis and associated with known expression of oestrogen receptor (ER), basal/myoepithelial cell markers, proliferation and survival. Negative staining for CK19 was present in 23.5% of tumours and was associated with histological type, grade, invasiveness, Ki67 index and expression of basal/myoepithelial cell markers. Positive staining for CK19 was associated with ER expression. Reduced expression of CK19 was associated with both shorter overall and disease-free survival rates; however, CK19 was not an independent prognostic factor in multivariate analysis. Reduced expression of CK19 in canine mammary carcinomas is related to an aggressive phenotype and may play a role in tumour progression.