Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of immature cells predominantly found in the bone marrow. In healthy mice, these cells represent about 0.5% of blood cells and less than 4% of splenocytes [1,2]. However, studies have shown that MDSCs accumulate in secondary lymphoid organs in many pathological conditions. In the particular case of cancer, MDSCs accumulate in the blood, spleen, lymph nodes and in the tumor microenvironment, where they suppress the immune response through various mechanisms, leading to tumour escape [3]. The aim of this study was to compare the percentage of circulating MDSCs in dogs with cancer (group 1), healthy dogs (group 2) and dogs with non-neoplastic diseases (group 3). For this, peripheral blood mononuclear cells were isolated, labeled with CD11b, CD14, MHCII antibodies and quantified by flow cytometry. Percentages of circulating MDSCs were calculated among the CD11b+ cells. Thirty dogs were included in group 1, 30 dogs in group 2 and 15 in group 3. The MDSCs median [IQR] value was 20.9% [10.8;35.8] in group 1, 2.9% [1.0;9.5] in group 2, and 15.9% [7.1;31.8] in group 3. Group 1 had significantly higher percentage of MDSCs compared to group 2 (p<0.01). There was no significant difference between group 1 and 3 (p=0.36). Our results showed a significant increase of circulating MDSCs in dogs with cancer compared to healthy dogs. Therefore, MDSCs could be an interesting biomarker for the diagnosis and follow-up of dogs with cancer. However non-neoplastic diseases were also associated with an elevated percentage of MDSCs. Thus, further studies are needed in order to assess the percentage of MDSCs as a specific tool for the early diagnosis of cancer and metastasis. [1] D.I. Gabrilovich, S. Nagaraj, Nat Rev Immunol 2009 ; [2] P. Trikha, W. Carson, Biochim Biophys Acta 2014. [3] K.H. Parker, et al., Adv Cancer Res 2015.