Acute phase proteins (APPs) are proteins whose concentrations in serum change after any inflammatory stimulus or tissue damage. The aim of the current study was to evaluate 3 positive APPs (C-reactive protein, serum amyloid A, and haptoglobin) and 1 negative APP (albumin) in female dogs with mammary neoplasia. Acute phase proteins were studied in 70 female dogs aged 8-12 years in the following groups: healthy (n = 10); mammary tumors in stages I (n = 19), II (n = 5), III (n = 6), IV (n = 5), and V (n = 7); and with mammary neoplasia plus a concomitant disease (n = 18). In animals with mammary neoplasia, significant increases of positive APPs were only detected in those that had metastasis or a neoplasm with a diameter greater than 5 cm and ulceration. Dogs with mammary neoplasia and a concomitant disease also had high C-reactive protein concentrations. Albumin concentration was decreased in animals with metastasis and with a concomitant disease. The results of the present study indicate that the acute phase response could be stimulated in female dogs with mammary gland tumors because of different factors, such as metastasis, large size of the primary mass, and ulceration or secondary inflammation of the neoplasm.