A 12-year-old bitch, Yorkshire terrier, was admitted to the Chungbuk National University animal hospital with draining from mammary masses in the left and right mammary glands. She was involved in the mammary masses, suffering bilateral inguinal hernia. Physical examination during the hospitalization demonstrated the masses in right (3 cm) and left (1.5 cm) mammary glands. Blood pool images didn`t show the subtle increase in blood flow to the inguinal hernia, and the increase of radionuclide distribution to the mammary gland on the soft tissue phase. However, delayed bone images showed a very focal lesion in the right mammary gland. These findings were confirmed on a subsequent biopsy, which also revealed adenocarcinoma with areas of malignant new bone formation. We present a case of primary mammary adenocarcinoma with $^{99m}Tc-MDP$ activity relative to normal bone. Tumor in the bitch demonstrates malignant new bone formation as the likely mechanism for the marked radiotracer avidity.