The 2-year prognosis of malignant mammary tumors seen in 175 bitches in the Tokyo metropolitan area was assessed based on their TNM clinical staging and histological classification. The larger the tumor size became (T category), the poorer was the clinical prognosis. The 2-year survival rates of the animals with regional lymph node metastasis of tumor cells (N1, N2 category) and/or distant metastasis (M1 category) were markedly lower than those of the animals without such involvement. As the grade of TNM staging increased, the prognosis was poorer, however, there were no significant differences in survival rates among subtypes of adenocarcinomas (tubular, papillary and papillary cystic) determined by WHO histological classification. It was also noticed that animals having carcinomas without tubular formation or myoepithelial cell proliferation had a lower survival rate than animals having carcinomas with those characteristics; and invasive carcinomas into adjacent skin or lymphatic/vascular vessels implied a poorer prognosis than non-invasive ones. The results suggest that a combined practice of TNM system and our evaluation on the above-mentioned 4 histologic features could be useful for prognostic determination of canine mammary cancers.