Domestic shorthair 10-year-old cat, not sterilized, not vaccinated, feeding from the table. We went to the clinic with breast cancer. General and special research methods were performed: blood analysis, x-ray examination of the lungs and abdominal ultrasound. Based on anamnesis and clinical examination, the diagnosis was made: a malignant breast tumor. The operation was performed. The operating material is sent for histological examination. Chemotherapy was prescribed. After the fi rst chemotherapy and monitoring of the animal's condition, it was necessary to perform a second unilateral mastoectomy of the remaining mammary glands, but the animal's owners refused to perform the operation. Chemotherapy was performed twice. Re-applied, after a year and a half with a sharp deterioration in the General condition and the appearance of a new tumor on the non-removed mammary glands. The owners refused the necessary treatment and decided to euthanize the cat. At the autopsy, breast neoplasms are presented as tubercles of various sizes. There are pronounced signs of inflammation, ulceration and necrosis foci. In the chest cavity, the lungs are enlarged, there were single and multiple formations of white and brown color, round shape of different diameters, dense consistency. There is a large number of metastases in the liver parenchyma. The organ is enlarged in size, dark brown in color, irregularly colored, with an uneven (bumpy) surface. In General, there are many dense nodes of a round-oval shape of white color, of various diameters. Histological examination of the tumor revealed foci of necrosis, atypical glandular complexes with frequently occurring mitosis figures. This structure indicates a low-grade breast adenocarcinoma. In the lungs, there is an expansion of the alveoli and bronchioles, thinning and rupture of the interalveolar partitions. There is a complex of tumor cells. In the liver, there is a lack of structural units of the liver, hepatocytes disperse chaotically, liver triads do not have a clear localization, atrophy and necrosis are expressed. Metastases in the liver, various forms. In a cat, a breast tumor interpreted as an adenocarcinoma had different biological behavior. At the initial diagnosis-adenocarcinoma of medium differentiation, and a year and a half later-low-grade adenocarcinoma.