Tumor malignancy is closely associated with cancer prognosis. However, each malignant tumor can have considerably different prognosis. Therefore, we aimed to characterize the factors that increase the malignancy of tumors by analyzing the associations between factors associated with tumor malignancy and prognosis and histological parameters of tumor malignancy. Ki-67, p53, p63, bcl-2, UGT-8 antibody expression was detected in malignant tumor of canine by immunohistochemistry. The results showed expression of Ki-67 was associated with histological subtypes of tumor, histological grade, and lymphatic invasion (P<0.05). Expression of p53 in the nucleus was only shown in inflammatory carcinoma types and significantly associated with histological type, high histological grade, and lymphatic invasion (P<0.05). High expression of p53 including cytoplasmic expression was shown to be negatively associated with central necrosis and lymphocyte infiltration (P<0.05). Expression of p63 was associated with molecular subtype, and Bcl-2 was shown to be associated only with central necrosis in tumors (P<0.05). But no relationship was found between UGT-8 expression and histopatholgical characteristics in malignant canine mammary tumors. In the canine malignant mammary tumors, we found that Ki-67 can be used as a favorable prognostic marker. But other markers or indices were not useful in evaluating malignancy of tumors.

Key words: Canine, Malignant, Mammary, Prognosis, Tumour