T-bet + lymphocytes infiltration as an independent better prognostic indicator for triple-negative breast cancer Abstract Purpose T-box transcription factor 21 (T-bet), which is the master regulator of effector T-cell activation, is derived by stimulation of T-cell receptors. In this study, we focused on T-bet and examined the function of activated T cells. Methods This study included 242 patients with primary triple-negative breast cancer (TNBC) who underwent resection without neoadjuvant chemotherapy between January 2004 and December 2014. The immunohistochemistry scoring for CD8 and T-bet expression on tumor-infiltrating lymphocytes (TILs) was defined as ≥ 30 per 6.25 × 10−3 mm2. Results Of the 242 TNBC cases, CD8 was positively expressed in 127 (52.5%) tumors, and T-bet was positively expressed in 67 (27.7%) tumors. T-bet expression was significantly correlated with CD8 expression (p < 0.0001). Patients with T-bet+ tumors had longer overall survival (OS) compared with patients with T-bet− tumors (p = 0.047). The combination of CD8+ and T-bet+ was associated with a better recurrence-free survival (RFS) and OS compared to CD8+/T-bet− tumors (p = 0.037 and p = 0.024, respectively). Adjuvant chemotherapy provided significantly greater benefit to patients with T-bet+ tumors (p = 0.031 for RFS, p = 0.0003 for OS). Multivariate analysis revealed that T-bet expression on TILs was an independent and positive prognostic indicator (HR = 0.36, 95% confidence interval (CI) 0.12–0.94, p = 0.037 for RFS, HR = 0.30, 95% CI 0.07–0.95, p = 0.039 for OS). Conclusions OS was significantly improved for patients with high T-bet-expressing TILs in TNBC. Thus, T-bet may be a predictive indicator for survival and various immunotherapy strategies in TNBC.