To determine the relationship between inflammatory markers and pathological complete response (pCR) in patients with locally advanced rectal cancer (LARC), who received neoadjuvant chemoradiotherapy (NACRT).Descriptive study.Department of Medical Oncology, Dr. Abdurrahman Yurtaslan Oncology Training and Research Hospital, Ankara, Turkey from January 2014 to June 2020.Patients older than 18 years of age, who underwent NACRT with a diagnosis of LARC, and who had no disease or drug-use that could affect inflammatory parameters, were included in the study. Inflammatory indices (neutrophil-to-lymphocyte ratio-NLR, platelet-to-lymphocyte ratio-PLR, lymphocyte monocyte ratio-LMR, systemic immune-inflammation index-SII, prognostic nutritional index-PNI) and changes in these indices, were calculated from blood samples taken before NACRT and before surgery. The relationship between pCR and calculated inflammatory indices was evaluated by comparing patients with and without pCR.Out of the 932 patients, who received NACRT with a diagnosis of LARC, 188 were eligible for the study. Median values of baseline SII for pCR and non-pCR groups were 729.3 (595.4-894.8) and 869.9(567.2-1145.2, p=0.049). Baseline NLR and PLR levels were lower in the pCR group than the non-pCR group in univariate analysis with a tendency to statistical significance. In the logistic regression analysis, which included NLR, PLR, and SII, only SII <748 was found to be an independent predictive factor of pCR (OR: 0.471, 95% CI; 0.224-0.991, p=0.047).Baseline SII might be an independent predictive factor for pCR in patients receiving NACRT with a diagnosis of LARC. Key Words: Locally advanced rectal cancer, Neoadjuvant chemoradiotherapy, Pathological complete response, Inflammatory index, Systemic immune-inflammation index, SII, NLR, PLR, LMR, PNI.