Mammalian sterile 20-like kinase 1 (Mst1) is a major inhibitor of cell proliferation, and is involved in apoptosis, oncogenesis and organ growth via its ubiquitously encoded serine threonine kinase. Previous studies have demonstrated that Mst1 has a tumor suppressor function in human breast cancer. Mst1 deletion or mutation is associated with tumorigenesis, whereas Mst1 overexpression leads to tumor cell apoptosis and decreases proliferation of tumor cells. Our previous study reported the tumor suppressive function of Mst1, and debated Mst1 as a prognostic factor in human breast cancer. In the present study, Mst1 levels were measured in the plasma of patients in order to elucidate their association with overall and disease-free survival. The results of the present study indicated that Mst1 is a strong prognostic and predictive factor in human breast cancer and a promising anticancer target.