Mammary tumors, with mastitis are the most important disease of the mammary glands. Neoplasia of the mammary glands in cats are in the third place in frequency. Risk factors for the development of mammary tumors in cats are animals age, breed and hormonal influence. Approximately 80 to 90% of mammary tumors are malignant, adenocarcinomas in the highest percentage. They are extremely aggressive as rapidly grow and metastasize to regional lymph nodes, lungs, liver and spleen, and are usually fatal ending. For most cats after surgery will develop a recurrence and will die within a year. The most important prognostic factors in estimating the time with no signs of disease and survival time after the operation is the size or volume of the tumor and the type of surgery. What is a nodule at the time of surgery less, it is better postoperative forecast. It is assumed that sex hormones play a role in the development of mammary tumors, because the majority of these tumors occur in the female, early ovariectomy significantly reduces the risk of development of such tumors and the use of synthetic progestins increases the risk. Radical unilateral or bilateral mastectomy was recommended surgical method for the treatment of malignant tumors of the mammary glands cat because it significantly reduces the likelihood of local recurrence. The combination of surgery and chemotherapy provides a much better prognosis than just surgery.