Mammary gland tumors are one of the most common neoplasms in bitches. They can be both, benign and malignant, but benign can alter into malignant. Malignant tumors have a poorer prognosis due to a greater chance of recidivism and the metastasis occurrence. Histopathological analysis and the degree of malignancy significantly influence the choice of treatment method and prognosis. The aim of this study was to investigate the incidence of mammary gland tumors in bitches presented at the Clinic of Obstetrics and Reproduction, Faculty of Veterinary Medicine, Zagreb and to determine tumor characteristics according to age, breed, values of hematological and biochemical parameters, size, type and degree of malignancy of the tumor. The research processed archival data collected during the regular clinical work of the Clinic of Obstetrics and Reproduction, Faculty of Veterinary Medicine, University of Zagreb, from 2016 to 2018. The data analyzed included the breed of animal, age, trias, hematological and biochemical parameters of blood, presence of metastatic changes, size and motility of the tumor, histopathological diagnosis and degree of malignancy. In our study, the average age was 10.5 years and 30% of patients were cross-breed bitches. The percentage of malignant mammary tumors was 87.8%, most commonly present in the caudal abdominal (27.7%) and inguinal mammary gland (39%). The proportion of tumors of malignancy degree I was 33.9%, II 46.8%, and III 19.3% . Degree of malignancy III was highest in tumors above 5 cm in diameter. A higher proportion of grade III tumors were found in inflammatory, ulcerated and tumors with affected lymph nodes. The proportion of nonsegmented neutrophils was increased in 53% of bitches, and the C-reactive protein in 41.2% of cases. In particular, these values were elevated in inflammatory and ulcerated tumors. The results of this study confirm that mammary tumors are one of the major health problems in bitches, especially in small breeds and in older bitches. Based on the research, we can conclude that the size of the tumor, inflammation, involvement of the lymph node and ulceration on the tumor surface significantly influence the degree of malignancy and are important prognostic indicators.