Objective: Leukocytes are hypothesized to reflect the inflammatory tumor microenvironment. We aimed to validate their prognostic significance in a large cohort of patients treated with pre-operative radiation for locally advanced rectal cancer (RC). Results: From 2004 to 2015, 257 RC patients with available biological data underwent a pre-operative radiotherapy, with a median age of 66 years. The median rectal EQD2 was 49.2Gy. Most of patients experienced concurrent chemotherapy (n = 245, 95.4%), mainly with 5-FU (83.3%). Clear surgical margins (i.e. complete resection) were achieved in 234 patients (91.1%). A complete (Mandard TRG1: n = 35, 13.6%) or almost complete pathological response (Mandard TRG2: n = 56, 21.8%) were achieved in 91 patients (35.4%). With a median follow-up of 46.1 months, 8 patients (3.1%) experienced local relapse, 38 (14.8%) experienced metastases and 45 (17.5%) died. Elevated pre-radiation neutrophil to lymphocyte ratio (NLR > 2.8) was identified as an independent predictive factor of increased local relapse, of decreased progression-free survival and overall survival in multivariate analysis. Elevated NLR was marginally associated with incomplete pathological response in multivariate analysis, suggesting a possible value as a biomarker of radio-sensitivity. Conclusions: Pre-radiation NLR is a simple and robust biomarker for risk stratification in locally advanced RC patients undergoing pre-operative radiotherapy, and might select the subpopulation eligible to treatment intensification or to neoadjuvant chemotherapy. © Vallard et al.