Mammary adenocarcinoma was diagnosed in a 3-year-old Landrace sow with prolonged infertility, anorexia and progressive emaciation after parturition. Gross examination confirmed a large tumour in the left anterior mammary gland with metastatic nodules on the pleura and in the parenchyma of lung. Microscopically, the tumour consisted mainly of solid adenomatous proliferations with numerous mitotic figures. Irregular glandular structures, solid nests of polygonal tumour cells without polarity and nests consisting of glandular, cribriform and solid portions were evident in dense fibrous stroma. Immunostaining revealed keratin in the tumour cells.