Background: Dilated cardiomyopathy is common in dogs. This form of cardiomyopathy is the main cause of death due toheart disease in dogs. Death can occur suddenly in clinically normal animals as a result of the progression of congestive heartfailure (CHF). The pathogenesis of heart failure syndrome in dogs with dilated cardiomyopathy involves activation of theneurohumoral system and immune-mediated inflammation, which leads to further progression of the condition. Heart failuresyndrome in dogs with dilated cardiomyopathy is caused by the progressive loss of cardiomyocytes, apoptosis, remodelingof the left ventricle, systolic and diastolic dysfunction, arrhythmias, reduced cerebral blood flow, the involvement of otherkey internal organs, and intestinal dysbiosis.Aim: This study aimed to determine the immunological and inflammatory mechanisms surrounding the development ofheart failure syndrome in dogs with dilated cardiomyopathy.Materials and Methods: The subjects of this study were dogs with a dilated form of cardiomyopathy (n=159), complicatedby various functional classes of heart failure syndrome. Evaluation of myocardial remodeling, systolic function, and systemichemodynamics was performed using EMP-860 Vet and PU-2200V ultrasound scanners according to the standard technique.Electrocardiography was performed with all dogs in right lateral recumbency using the EK1T-04 Midas electrocardiograph(50 mm/s speed and 1 mV gain = 1 cm).Results: In some affected animals, especially in cases of compensated dilated cardiomyopathy, leukocytosis was noted. Inpatients with dilated cardiomyopathy complicated by heart failure syndrome of various functional classes, the number ofneutrophils was significantly increased, and the number of lymphocytes was decreased by 1.9-2.1 times when comparedwith those in clinically normal animals. In dogs with dilated cardiomyopathy, neutrophilic leukocytosis develops with asimple regenerative shift to the left. The results of immunological studies indicate that dogs with dilated cardiomyopathydevelop T lymphocytopenia as compared with clinically normal animals.Conclusion: The central component of heart failure syndrome in dogs with dilated cardiomyopathy is the activation of theneurohumoral system and immune-mediated inflammation. The development of CHF in dogs with dilated cardiomyopathyis caused by the progressive loss of cardiomyocytes, apoptosis, remodeling of the left ventricle, systolic and diastolicdysfunction, arrhythmias, reduced cerebral blood flow, involvement of other key internal organs, and intestinal dysbiosis.Keywords: dilated cardiomyopathy, dogs, pathogenesis, heart failure, immunity and inflammation.