We investigated the expression of bcl-2, estrogen receptor alpha (ER-alpha), caspase-3, -8, -9, proliferating cell nuclear antigen (PCNA) and Ki-67 in canine mammary carcinomas. We used 65 paraffin embedded and re-diagnosed archival canine mammary tumor samples to which we applied the routine streptavidin-biotin-peroxidase technique. Seventeen cases were re-diagnosed as tubulopapillary carcinoma, 31 were re-diagnosed as complex carcinoma and 17 were re-diagnosed as carcinosarcoma. Differences of expression of bcl-2 and PCNA were statistically significant according to tumor type. Differences in expression of ER-alpha, caspase-3, -8, -9 and Ki-67 were not statistically significant. Differences of expression of bcl-2 and PCNA were statistically significant compared to ER-alpha, caspase-3, -8, -9 and Ki-67 in carcinosarcomas. We report the prognostic significance of bcl-2 and PCNA expression in canine mammary carcinosarcomas.