A 14-year-old virgin, female Yorkshire Terrier showed enlarged abdomen after udderectomy was diagnosed with canine mammary gland tumor accompanied with hydrometra. Gross findings appeared a white-yellowish <TEX>$l0{ imes}7{ imes}7cm sized solid mass containing cysts filled with various colored mucoid fluids in the udder. Both uteri were distended greatly with white-yellowish watery fluids and their serosal surfaces were translucent. Characteristic micyoscopic findings in the mass of the udder showed great expansion of the mammary lobule and stroma and complete replacement of the glandular elements by prolifaration of myoepithelial cells. The mucosa of the uterus showed severe atrophy of the uterine epithelium.