Through targeting IL-25, an IL-17 family cytokine that positively regulates the initiation of type 2 immune response, we found that anti-IL-25 blocking antibody treatment led to more than 50% reduction of pulmonary metastasis in a spontaneous breast tumor model—the MMTV-PyMT transgenic mice, accompanied with reduced Th2 and M2 cells in the primary tumor tissue. Here, IL25 is linked to breast neoplasm.