BackgroundAlthough human studies have shown that inflammation plays a role in the development of congestive heart failure, scarce information exists on white blood cell count (WBC) and differential cell counts in various stages of heart failure in man and dogs. A few studies demonstrated increased concentrations of C-reactive protein (CRP), a major acute-phase protein, in cardiac diseases in dogs. Our research aimed to investigate whether CRP concentration, WBC and neutrophil count (NEUT), as markers of systemic inflammation, are elevated in canine cardiovascular patients. We also aimed to find out whether there is an association between CRP concentration and WBC and NEUT, as well as associations between these inflammatory markers and selected echocardiographic parameters. Sixty-two client-owned canine cardiac patients and 12 healthy dogs were included in the study. The patients were classified into International Small Animal Cardiac Health Council classes (ISACHC I–III). The serum CRP concentration was determined using a canine CRP test kit. WBC and NEUT were determined using an automated hematology analyzer.ResultsSignificantly higher serum CRP concentration, WBC and NEUT were found in the decompensated stage of heart failure (ISACHC III) compared with healthy dogs and with patients in ISACHC group II and ISACHC group I. Serum CRP concentration significantly positively correlated with WBC (r = 0.65, P < 0.001) and NEUT (r = 0.58, P = 0.002) in the ISACHC III group, while no significant correlations were found in the ISACHC I and II groups. A significant negative correlation between serum CRP concentration and the left ventricular ejection fraction (r = − 0.49, P = 0.046) and a significant positive correlation between CRP and the E wave velocity of the mitral valve inflow (r = 0.52, P = 0.046) were found in the ISACHC III group.ConclusionsThe CRP concentration, WBC and NEUT were significantly increased in advanced-stage heart failure patients in comparison with compensated patients and healthy dogs, which indicate the presence of systemic inflammation. However, normal CRP concentration and normal WBC and NEUT can also be present in heart failure.Electronic supplementary materialThe online version of this article (10.1186/s13028-018-0372-x) contains supplementary material, which is available to authorized users.