Background: A consensus interpretation of the IDEA colon cancer study results suggested that risk categories based on T/N stage grouping be used to guide decision-making for duration (3 vs 6 months) of adjuvant FOLFOX or CapeOX chemotherapy. Given the prognostic potential of immune biomarkers, we examined the immunoscore and individual T and B lymphocyte markers in low and high risk T/N subsets of stage III colon carcinoma patients (N=600) treated with adjuvant FOLFOX. Methods: Immunoscore (CD3⁺, CD8⁺) and individual T-cell and CD20⁺ B-cell immunostain densities in central tumor (CT) and invasive margin (IM) of FFPE sections were quantified by image analysis. A predetermined immunoscore categorization was used [high (2-4) vs low (0-1)]. Individual markers were analyzed by backwards selection wherein CD3⁺ IM was most robust for prognosis and an optimized cutoff was then determined. Associations with disease-free survival (DFS) were analyzed by multivariable Cox regression adjusting for age, T/N stage, sidedness, KRAS/BRAF, and DNA mismatch repair. Results: In low and high risk T/N patient subsets, the immunoscore and CD3⁺ IM were each significantly discriminant for prognosis. Among low risk (T_1-3N₁) patients, a high vs low immunoscore was associated with a 91% vs 77% 3-year DFS [HR 0.57, 95% confidence interval (CI) 0.34-0.95, adjusted (adj) P= 0.026]. Among high risk (T₄ or N₂) patients, a high vs low immunoscore was associated with a 68% vs 54% 3-year DFS (HR 0.64, 95% CI 0.42-0.98, P_adj= 0.034]. Similarly, a high vs low intratumoral CD3⁺ density at the invasive margin (IM) was significantly associated with prognosis in low risk [HR 0.37, 95% CI 0.21- 0.66), P_adj< 0.0003] and in high risk [HR 0.47, 95% CI, 0.27- 0.80), P_adj< 0.0028] patient subsets. Conclusions: Immunoscore and CD3⁺ IM were shown to prognostically stratify FOLFOX-treated patients within both low and high risk T/N subsets. These data underscore limitations of the T/N risk classification for adjuvant treatment decisions in stage III patients, and demonstrate the ability of T-cell markers to enhance prognostication to guide clinical decision-making.