AbstractThe aim of this study was to investigate immune response and its prognostic significance in colon carcinomas using the previously described Immunoscore (IS). A population‐based series of 779 colorectal cancers, operated on between 2000 and 2010, were classified according to tumour, node, metastasis (TNM) status, mismatch repair (MMR), and BRAF mutation status. Rectal cancer cases (n = 203) were excluded as a high proportion of these patients received preoperative neoadjuvant chemoradiotherapy. Tissue microarray (TMA) samples collected from the tumour centre and invasive front were immunostained for CD3 and CD8. Lymphocytes were then digitally calculated to categorize IS from grade 0 to 4. Samples adequate for IS were available from 510 tumours. IS was significantly associated with AJCC/UICC stage, T stage, lymph node and distant metastases, perineural and lymphovascular invasion, MMR status, and BRAF mutation status. For IS0, IS1, IS2, IS3 and IS4, respectively, the 5‐year disease‐free survival (DFS) rates were 59, 68, 78, 83 and 94% (p < 0.001); 5‐year disease‐specific survival (DSS) rates were 47, 55, 75, 80, and 89% (p < 0.001); and 5‐year overall survival (OS) rates were 40, 44, 66, 61, and 76% (p < 0.001). IS was also prognostic for DFS, DSS, and OS within subsets of microsatellite‐stable (MSS) and microsatellite‐instable (MSI) disease. Multivariable analysis showed that IS, AJCC/UICC stage, lymphovascular invasion, and lymph node ratio in AJCC/UICC stage III disease were independent prognostic factors for DFS, DSS, and OS. Age was an independent prognostic factor for DSS and OS. Gender and BRAF mutation were independent prognostic factors for OS. In conclusion, IS differentiated patients with poor versus improved prognosis in MSS and MSI disease and across AJCC/UICC stages. IS, AJCC/UICC stage, lymphovascular invasion, and lymph node ratio in AJCC/UICC stage III disease were independent prognostic factors for DFS, DSS, and OS.