We concentrated ourselves to evaluate the prognostic significance of the p53 gene mutations, its protein expression and MIB-1 index as a proliferative marker in canine mammary tumors. In the present study, a total of 20 cases were examined, among which there were 5 malignant mixed tumors, 4 mammary gland adenocarcinomas, 1 papillary adenocarcinoma, 8 benign mixed tumors and 2 mammary gland adenomas. Positive immunostaining for p53 with PAb240 antibody was found in 2 benign (20%) and 3 malignant (30%) tumors. However, PAb421 antibody did not give positive result at all. In Western blot analysis, the p53 expression in benign and malignant tumors was detected in 4 and 3 cases, respectively. p53 mutations were found in 6 cases out of the cases with detected p53 protein expression. The MIB-1 index in benign and malignant tumors were 17.6 กพ 20.8% and 29.0 กพ 27.2%, respectively and there was no significant difference between tumor types. There was a significant correlation between p53 mutations and p53 overexpression (correlation coefficient = 0.5, p < 0.05). In Kaplan-Meier survival analysis, the p53 index was associated with significantly shortened survival time (p < 0.01). In multivariate analysis, p53 overexpression was only an independent factor for indicator of worse prognosis in canine mammary tumors (p = 0.01). These results demonstrated that p53 gene mutations and protein overexpression using the PAb240 anti-p53 antibody were useful predictors of increased malignant potential and poor prognosis in canine mammary tumors.