In this retrospective study, we investigated how a quantitative ELISA essay of NK cell activation-induced interferon-gamma (IFN-γ) secretion correlates with various proven clinicopathological factors of breast cancer at the time of diagnosis, as an alternative to the complex and time-consuming immunohistological evaluation of the role of tumor-infiltrating lymphocytes, which has previously shown conflicting results. The gene discussed is IFNG; the disease is breast cancer.