Mammary neoplasias are the most frequent tumors in female dogs and surgical excision is the recommended treatment. The objective of the present investigation was to assess the influence of the surgery type for mammary neoplasia excision - simple mastectomy or en bloc resection - in the clinical evolution of the treated animal. Thirty bitches presenting histologically confirmed malignant mammary tumors were used. They were all patients of the Veterinary Hospital of University Paulista, with ages ranging from 5 to 10 years and of various breeds. The dogs in this study did not have any concurrent treatment for their neoplastic disease and were divided into 2 groups - some were submitted to simple mastectomy and others, to en bloc resection. Before surgery, physical examination, biochemical profile and thoracic radiographs were performed. After surgical resection, these animals were regularly monitored for 2 years both clinically and by laboratory tests for neoplasms and the development of metastases. The average age at the time of tumor excision was 8.5 years and adenocarcinomas were the most common type. Higher prevalence of tumors occurred in animals of no defined breed, followed by German shepherds, Poodles and Dobermans. There was no positive correlation between occurrence of pregnancy or pseudopregnancy and the presence of mammary neoplasias. Additionally, there was no significant difference in the survival time of animals submitted to simple mastectomy or en bloc excision and this period ranged from 4 to 24 months. Female dogs submitted to en bloc resection presented more postoperative complications, such as infections, dehiscence of the surgery site, decubital ulcers, hypostatic congestion of pulmonary parenchyma and postoperative pain. It was concluded that the surgery type used for mammary tumor excision had no influence in the survival time of the patient, neither in metastasis appearance nor in local recurrence.