Ovarian remnant syndrome (ORS) represents the presence of functional ovarian tissue after routine castration of the bitch, caused either by surgeon error or the presence of anomalous ovarian tissue. The piece of ovary that remains after the surgery, may continue to develop and become functional, secreting hormones as estrogen and progesterone which cause signs of pro-estrus or estrus, even pseudocyesis. All those signs cause consternation among owners, who thought that the reproductive activity in their dog will be completely abolished. One mixed-breed, 6 years old bitch, 27.3 kg, spayed 8 months before, was presented with purulent vaginal discharge and mammary lumps, after the animal has been treated by her veterinarian with Proligestone to suppress the persisting estrus signs. Our investigations included vaginal cytology, vaginoscopy, the examination of the mammary chain, ultrasound, radiography, serum chemistry, blood count and histopathological examinations. The ultrasound revealed a filled uterine stump and the presence of a mass caudal the right kidney, which was suspected as an ovarian neoplasm. The examination of the mammary chain revealed masses in right M 3, M4, M5 glands and in the left M2 gland. The bitch underwent surgery, and the results of the histopathological examination were: dysgerminoma, pyometra, and the mammary masses were classified as carcinomas. In this paper we wanted to underline the fact that the Ovarian Remnant Syndrome may imply serious clinical consequences which can even lead to the death of the animal. We recommend as a method of treatment for this condition the surgical removal of the remnant tissue.