41 Background: Several studies have demonstrated a beneficial prognostic impact of tumour-infiltrating lymphocytes (TILs) in oesophageal and gastric adenocarcinoma, but whether this association differs according to adjuvant chemotherapy has not been reported. Herein, we examined the prognostic impact of different TIL subsets according to adjuvant chemotherapy in these cancer types. Methods: Immunohistochemistry was applied to assess the density of T cells (CD3+, CD8+, FoxP3+) and natural killer (NK)/T cells (CD56+) in radiochemonaïve tumors from a consecutive cohort of 174 patients with resected oesophageal or gastric adenocarcinoma. Cox proportional hazard’s modelling was applied to examine the impact of the investigated markers on time to recurrence (TTR) and overall survival (OS). Results: Dense infiltration of CD3+, CD8+, and FoxP3+ cells were all associated with a significantly prolonged TTR in univariable Cox regression analysis (hazard ratio[HR] = 0.57; 95% confidence interval [CI] 0.37-0.89; HR = 0.31, 95% CI 0.13-0.77, and HR = 0.44, 95% CI 0.28-0.68). This association remained significant for CD8+ and FoxP3+ cells in multivariable analysis, adjusted for age, tumour location, TNM stage, differentiation grade, resection margin and adjuvant chemotherapy ( HR = 0.30, 95% CI 0.11-0.77, and HR = 0.52, 95% CI 0.32-0.84), and borderline significant for CD3+ cells (HR = 0.63, 95% CI 0.39-1.02). Similar associations were observed for all markers in relation to OS. Notably, analysis in strata according to adjuvant chemotherapy revealed that none of the investigated lymphocyte subsets carried prognostic information in patients having received treatment (n = 13; 7.5%), with a significant treatment interaction between CD3+ and CD8+ cells and adjuvant chemotherapy (p^interaction0.035 and 0.021). Conclusions: These results confirm the prognostic value of a high density of TILs in gastroesophageal adenocarcinoma, but also indicate that adjuvant chemotherapy is only beneficial in patients with tumors displaying a low density of TILs. These results are of potential clinical relevance and need to be confirmed in additional, preferentially randomized, studies.