Background: Feline gingivostomatitis is a chronic disease of domestic cats characterised by inflammatory lesions along the gingiva and oral cavity. Objectives: The aim of this study was to investigate oral vascular endothelial glycocalyx damage in gingivostomatitis caused by viral infections and other causes in cats using selected biomarkers of endothelial damage. Methods: The material of this study consisted of 55 cats with gingivostomatitis and 15 healthy cats of different age, breed and sex. A total of 34 of 55 cats with gingivostomatitis had viral infections, whereas the cause of 21 cats could not be determined. Viral diseases were diagnosed by clinical findings and rapid antigen tests. Haemogram analysis was performed from blood samples. Serum endothelin-1 (ET-1), syndecan-1 (SDC-1), myeloperoxidase-ANCA (MPO-ANCA) and interleukin-6 (IL-6) biomarker concentrations were measured using feline-specific commercial ELISA kits. Cats were treated medically for gingivostomatitis. Results: Although 49 cats recovered with the treatment, 6 cats died. There was a significant increase in serum ET-1 and SDC-1 concentrations and no difference in serum MPO-ANCA and IL-6 concentrations in cats with gingivostomatitis. In addition, total leukocyte (WBC), granulocyte (GRA) and monocyte (MON) counts were increased in cats with gingivostomatitis. Conclusion: Serum levels of ET-1 and SDC-1, which are biomarkers of vascular endothelial damage, were increased in cats with gingivostomatitis. It was shown that vascular endothelial damage occurs in gingivostomatitis and that the biomarkers ET-1 and SDC-1 can be used to detect this damage and have a reliable diagnostic value.