Medical records of fifty-three cats diagnosed as malignant mammary tumor from 1982 to 1993 were reviewed. The mean age of the cats at diagnosis was 11.1 years, and Japanese domestic and Siamese breeds were predominant. Survival rates after 1 and 2 years of diagnosis were 31.8% and 17.7%, respectively, suggesting poor prognosis of this malignancy. The survival time was significantly associated with tumor size or WHO clinical stage, but not with breed, age, or gender. Twenty-nine cats with pulmonary metastasis died within 5 months following metastatic detection. Postoperative prognosis was significantly related to the tumor size at surgery, but not with type of surgery and adjuvant chemotherapy using cyclophosphamide and/or vincristine.