Background: Tumor-infiltrating lymphocytes (TILs) are significantly associated with clinical outcomes in patients with colon cancer. However, the potential for a differential effect of TILs on prognosis based on primary tumor sidedness has not been studied. We determined the interaction between TILs and tumor sidedness in relationship to disease-free survival (DFS). Methods: We analyzed data on TIL densities by primary tumor sidedness in 1532 stage III colon carcinomas from participants in a phase III trial of FOLFOX-based adjuvant chemotherapy (NCCTG N0147). TIL densities were dichotomized as low or high (≤3 /HPF) based on an optimized cutoff previously identified for DFS in this cohort. Right-sided tumors were defined as proximal to the splenic flexure. Analysis of TILs and sidedness with DFS were examined using Kaplan-Meier methodology and multivariable Cox regression. Results: Overall, tumors with high vs low TILs had the best DFS [HR_adj: 0.58 (95%CI: 0.45-0.74); P_adj<0.0001]. The association of TIL densities with 5-yr DFS differed significantly by primary tumor sidedness (P_{interaction (adj)} = 0.045). Among right-sided tumors, high vs low TILs were significantly associated with improved DFS (P_adj<0.0001)[Table]. Among left-sided tumors, however, DFS did not differ significantly for high vs low TILs (P_adj =0.173)[Table]. Similar results for TILs and DFS by sidedness were found for pMMR cancers. We then analyzed our data in low risk (T_1-3, N₁) and high risk (T₄ and/or N₂) tumors. Among low risk tumors, high vs low TILs was significantly associated with improved 5-yr DFS only in right-sided tumors (P_adj =0.006) [Table]. Among high risk tumors, high vs low TILs were significantly associated with better DFS in both right-sided (P_adj<0.001) and left-sided (P_adj =0.024) tumors. Conclusions: Overall, tumors with high TIL densities had significantly better DFS in right-sided but not left-sided cancers. Among low risk patients, the association of high TILs with better DFS was limited to right-sided tumors. These findings suggest that TILs should be interpreted by sidedness for prognostication. ClinicalTrials.gov Identifier: NCT00079274.