Stress has been considered to negatively affect the outcome of cancer. In the physiological stress response neurotransmitters and hormones modulate immune cell function that in turn alters the tumor environment, thus, modulates tumor growth. In particular, myeloid-derived suppressor cells (MDSC) that were initially identified in tumor patients and are characterized by the expression of CD11b, Ly6G and Ly6C seem to play a critical role in the promotion of tumor growth. As the increase in CD11b-positive cells in general is a hallmark of exposure to stress we sought to investigate the impact of chronic psychosocial stress on MDSC development and subsequent tumor growth. We applied chronic subordinate colony housing (CSC), a previously established paradigm for chronic psychosocial stress. We analyzed the phenotype and function of CD11b-positive myeloid cells accumulating in stressed mice as well as those arising in response to an external tumor. After 19days of CSC myeloid cells were increased in blood and secondary lymphoid organs. Detailed analysis of such cells revealed their MDSC-like phenotype. Consistently, we observed enhanced tumor-growth in stressed mice. Previous exposure to CSC enhanced suppressive activity of MDSC in tumor-bearing mice. In conclusion, our data indicate that exposure to chronic psychosocial stress induces the development of MDSC, that gain functional activity in tumor-bearing mice, thus, promoting tumor growth.