Abstract Background: Tumor-infiltrating lymphocytes (TILs) at diagnosis are reported to be prognostic in triple-negative breast cancer (BC). Analysis of a small subset of 209 HER2+ patients (pts) with 49 events concluded that higher levels of S-TILs are associated with increased trastuzumab benefit (Loi, 2014). Here we report the largest study to date evaluating S-TILs and their prognostic and predictive association with clinical outcome in N9831 pts treated with either chemotherapy or chemotherapy plus trastuzumab. Methods: Samples assessed were from primary tumors of pts on N9831 arm A (standard AC→T chemotherapy) and arm C (concurrent chemotherapy with trastuzumab) (Perez, 2011). S-TILs were evaluated on H&E whole tumor slides by a single pathologist with ∼10% of cases read by two pathologists in tandem. The percent of stromal lymphocytic infiltrates (S-TILs) was quantitated in deciles; ≥60% S-TILs was used for the categorical cutoff (Denkert, 2010). The association between S-TILs, treatment (tx) and recurrence-free survival (RFS) was studied and the interaction between S-TILs, trastuzumab benefit and RFS was calculated. Results: 489 pts from arm A (chemo) and 456 pts from arm C (chemo with trastuzumab) were assessed and were similar to pts in the overall trial; all had RFS information and a median follow-up of 4.4yr. Tumors from 54% of pts in arms A and C were HR+; 14% were node-negative. Tumors with high S-TILs were more likely to be hormone receptor-negative (p< 0.0001). In multivariable analyses including nodal status, hormone receptor status, tx arm, tumor size, tumor grade, and age, ≥60% S-TILs was significantly associated with RFS (HR 0.20; 95%CI 0.064–0.65, p=0.007) in arm A but not in arm C (HR 1.1; 95%CI 0.42–2.8, p=0.87); the interaction term of arm and ≥60% S-TILs was significant (p=0.042). Semi-continuous deciles were associated with RFS in arm A (p<0.0002) but not in C (p=0.37). Hormone receptor status was an independent prognostic factor in arm A (HR 0.61; 95%CI 0.41-0.93, p=0.02) but not in C (HR 0.79; 95%CI 0.44-1.41, p=0.42). In arm A the 10yr Kaplan-Meier estimates for RFS were 90.9% and 64.5% for high S-TILs and low S-TILs pts, respectively (HR 0.23; 95%CI 0.073–0.73, p=0.013). In arm C the 10yr Kaplan-Meier estimates for RFS were 80.0% and 80.1% for high S-TILs and low S-TILs pts, respectively (HR 1.26; 95%CI 0.5–3.2, p=0.63). Arm A (N=489)Arm C (N=456)VariableHR (95% CI)p-valueHR (95% CI)p-valueNodal statusLymph node(-)1.00 (ref)<0.00011.00 (ref)0.0131-3+0.58 (0.25, 1.35) 2.99 (0.40, 22.70) 4-91.46 (0.64, 3.35) 3.29 (0.42, 25.80) 10+2.42 (1.04, 5.64) 8.22 (1.06, 63.60) HR statusnegative1.00 (ref)0.01981.00 (ref)0.42positive0.61 (0.41, 0.93) 0.79 (0.44, 1.41) S-TILs status<60%1.00 (ref)0.0071.00 (ref)0.87≥60%0.2 (0.06, 0.65) 1.08 (0.42, 2.79) Tumor gradeGrade 1 or 21.00 (ref)0.501.00 (ref)0.51Grade 31.18 (0.73, 1.91) 1.24 (0.65, 2.39) Tumor size1.08 (0.98, 1.19)0.121.00 (0.93, 1.07)0.92Age0.99 (0.97, 1.00)0.131 (0.98, 1.03)0.87 Conclusions: In exploratory analyses from this subset HER2+ population from N9831, S-TILs were associated with RFS in patients treated with chemotherapy alone, and were not shown to be associated with RFS in patients treated with chemotherapy plus trastuzumab. Citation Format: Edith A Perez, Karla V Ballman, S Keith Anderson, E Aubrey Thompson, Sunil S Badve, Helen Bailey, Frederick L Baehner. Stromal tumor-infiltrating lymphocytes(S-TILs): In the alliance N9831 trial S-TILs are associated with chemotherapy benefit but not associated with trastuzumab benefit [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr S1-06.