Mammary tumours are the second most frequent neoplasia in dogs, mainly affecting older female patients. Approximately 50% of the mammary tumours are malignant with high percentage of mortality, if not treated in time. The aim of this study was to analyze the data of canine patients with mammary tumours, to evaluate the type of tumours, as well as the relationship between tumour incidence and dogs’ age, reproductive cycle and sterilization. The survey was used to retrieve the information in the period of two years from the patient data base of the University Veterinary Hospital at the Faculty of veterinary medicine in Skopje. Patients included in this survey were subjected to routine clinical investigation and additional laboratory tests (cytological examination, x-rays imaging, CBC and biochemical profile, histopathology of the tumor samples). Aged female patients (12 - 13 years) are the most susceptible category for development of mammary tumours. The reproductive history showed that five of the patients with malignant mammary tumours have never whelped and were not treated with any exogenous hormones. Malignant tumours (adenocarcinoma) were diagnosed in 90% of the patients. Three patients died due to lung metastasis. Late diagnosis is one of the major problems that results in lethal outcome due to lung metastases. Since ovarian steroids play an important role in the aetiology, the most effective prevention of mammary tumours is elective ovariectomy of the bitch at an early age.