Surgery remains the treatment of choice for female dogs with mammary gland tumors. Chemotherapy is not commonly used as an adjuvant therapy. Cyclooxygenase 2 (COX-2) has been related to angiogenesis development in tumors, disease progression and worse prognosis. The aim of this prospective study was to compare overall survival periods of female dogs diagnosed with advanced mammary tumors submitted to different treatment protocols, including surgery, chemotherapy and cyclooxygenase inhibitors. Twenty-nine female dogs were evaluated and treated with four different protocols. The overall survival of patients with low COX-2 scores was longer when compared to patients with high COX-2 scores. Different proposed adjuvant treatments associated with surgery led to a statistically significant longer overall survival when compared to surgical treatment alone. Canine patients presenting malignant mammary gland neoplasms with advanced clinical staging should be submitted to complementary therapeutic medication based on clinical staging and immunophenotypical characteristics of the disease.