The present study was conducted to investigate the antioxidant status in canine cancer patients. Patients with multicentric lymphoma, oral fibrosarcoma, mast cell tumor, malignant melanoma, appendicular osteosarcoma, nasal tumors and peripheral ameloblastoma were selected. Each group consisted of 6 patients. Total antioxidant capacity (TAC) and enzyme antioxidants: glutathione peroxidase (GPX), catalase (CAT) and superoxide dismutase (SOD) were measured in serum and whole blood, respectively, and were compared to 31 healthy dogs. The results of the study showed a significant increase of CAT activity in tumor groups except in the patients with nasal tumors. SOD activity increased significantly in malignant melanoma, mast cell tumor, multicentric lymphoma and oral fibrosarcoma patients. Appendicular osteosarcoma and multicentric lymphoma patients showed significantly increased levels of GPX and TAC, respectively. Activities of CAT and SOD were significantly higher comparing the all 42 cancer patients with healthy dogs. Tumor patients showed significantly lower levels of haemoglobin when compared to healthy dogs. The increase of antioxidant enzyme activities and TAC in these animals suggest the activation of antioxidant defence mechanisms in different cancer diseases. Further studies involving more animals and other antioxidant parameters and oxidative stress markers are necessary.