Background: Tumor infiltrating lymphocytes (TILs) and tumor budding (linked to epithelial mesenchymal transition) may influence metastatic potential and patient prognosis. We analyzed these features and their relative contribution to survival among low (T_1-3 N₁) and high (T₄ and/or N₂) risk groups, defined by the IDEA study, used to inform the duration of adjuvant chemotherapy in stage III colon cancer. Methods: Among 1,532 patients (low risk n=804; high risk n=728) treated in a phase III adjuvant trial of FOLFOX + cetuximab (x 6 months), intraepithelial TIL densities and tumor budding were quantified at microscopy in routine histologic sections. Optimal cutpoints were determined in association with 5-yr disease-free survival (DFS). Relative contribution of variables to DFS was calculated using χ2 from Harrell’s rms R package based on multivariable Cox regression models. Results: In the overall cohort, the tumor budding/TILs combined variable was more robust for predition of DFS than either alone. Budding/TILs was significantly associated with DFS in both low (HR_adj, 1.59; 95% CI, 1.02-2.48; p=.0273) and high (HR_adj, 2.82; 95% CI, 1.72-4.63; p<.0001) risk patients. We then determined its relative contribution (%) to DFS (Table). Among low risk, budding/TILs ranked second (24.4%) behind KRAS status (45.5%) and ahead of treatment arm (7.2%) and mismatch repair (MMR) status (6.1%). Among high risk, budding/TILs contributed the most to DFS (45.4%) followed by primary tumor sidedness (13.0%), performance status (12.0%), and MMR (10.4%). Conclusions: Tumor budding/TILs provides robust prognostic stratification by risk group to improve anatomic tumor staging. The relative contribution of budding/TILs to DFS was second only to KRAS status in low risk patients, and was the most important predictor of DFS in high risk patients. Evaluation in patients treated with 3 vs 6 mos of adjuvant chemotherapy is warranted.