A nine-year-old female Yorkshire terrier dog showing mammary secretion mixed with blood and pus for 5 month duration was presented. Cytologically mammary secretion consisted of many pleomorphic tumor cells, RBC, and neutrophil. Tumor cells were characterized by polyhedral pleomorphic nuclei with smudged chromatin and basophilic cytoplasm with many secretory vacuoles. N/C ratio was very high. Therefore it was diagnosed as mammary gland adenocarcinoma. Histological examination confirmed cytological diagnosis. From these results cytology isfor the diagnosis of secreting mammary gland adenocarcinoma is very simple and accurate for the diagnosis of secreting mammary gland adenocarcinoma.