Background: Rectal cancer constitutes over one-third of all colorectal cancers (CRC) and is one of the leading cause of cancer-related death in developed countries. Treatment modalities applied in locally advanced tumors differ substantially among research centers. In order to identify high-risk patients and better adjust the therapy new markers are needed. Systemic inflammatory response (SIR) markers such as LMR, NLR and PLR have been proved highly prognostic in many malignancies, including CRC; however, they lack proper validation. In our study we assessed the reproducibility of LMR, NLR and PLR. Methods: Sixty patients with locally advanced rectal cancer treated in Maria-Sklodowska Curie National Institute of Oncology in Warsaw, Poland between 08.2017 and 12.2020 were prospectively enrolled in the study. Three consecutive blood morphology tests of each patient within a median period of 21 days were obtained before start of the treatment. Results: LMR, NLR and PLR calculated at two time-points were correlated with the coefficient of 0.776, 0.696 and 0.751 (p < 0.005 in all measurements), respectively. Patients were divided into LMR, NLR, PLR-high and low groups. If LMR at the first test was out of the range of 2.2-3.0 (+/- 0.4 from the cut-off) the risk of misclassification in the second measurement defined as an affiliation to a different (high or low) group than initially was 5.0% (95% CI 1.0-13.9%). In case of NLR, when outside of the range of 2.5-3.5 (+/- 0.5) it was 8.3% (95% CI 2.8-18.4%) and PLR outside of the range of 125-175 (+/- 25) 10.0% (95% CI 3.8-20.5%). Mean percentage change between the third and the first measurement of lymphocytes, monocytes, neutrophils and platelets count ranged from -5.59% to 4.76% and the standard error from 2.0 to 3.9. Conclusions: In conclusion SIR markers are reproducible, easily obtained biomarkers with potential application in clinical practice.