Background: The aim of our study was to investigate the prognostic role of the neutrophil to lymphocyte ratio (NLR), derived neutrophil to lymphocyte ratio (dNLR), platelet to lymphocyte ratio (PLR), and lymphocyte to monocyte ratio (LMR) in PFS and OS of patients with neuroendocrine tumors (NETs) of the gastrointestinal tract. Methods: We analyzed medical records of 69 patients with stage I - IVB NETs of the gastrointestinal tract (25 men, 44 women; median age is 57 years [44.75-66.00]). All patients were treated according to the standard protocols from 2015 till 2020. We examined demographics, clinical stage, tumor morphology, and baseline levels of leukocytes, neutrophils, lymphocytes, monocytes, and PLT. We also analyzed the calculated value of NLR, dNLR, PLR and LMR. Results: The ROC analysis made it possible to determine that the factors, that have a significant impact on the PFS indicator are: absolute (cut-off: ≤2.26; p = 0.0274) and relative (cut-off: ≤30; p = 0.0158) lymphocyte count, NLR (cut-off: 1.85; p = 0.0230) and dNLR (cut-off: 1.40; p = 0.0209). The median PFS of patients whose baseline absolute lymphocyte count 2.26x109/l or less was 34.0 months and was significantly (61%) less than the median PFS in the group of patients with an absolute lymphocyte count greater than cut-off value (p = 0.0122; HR = 0.39: 95% CI 0.19-0.81). The median PFS of patients with relative lymphocyte count £30% was 25.0 months and was significantly (61%) less than the median PFS in the group of patients with lymphocyte count > 30% (p = 0.0082; HR = 0.39: 95% CI 0.20-0.79). The median PFS of patients with NLR > 1.85 was 26.0 months and was significantly (60%) less than the median PFS in the group of patients with NLR1≤.85 (p = 0.0095; HR = 0.40: 95% CI 0.21-0.80). The median PFS of patients with dNLR > 1.40 was 20.0 months and was significantly (60%) less than the median PFS in the group of patients with dNLR≤140 (p = 0.0120; HR = 0.40: 95% CI 0.20-0.82). In multivariant analyses, which contained all significance factors by univariate analysis, count of lymphocytes: lymphocytes≤30% significantly increased the risk of disease progression: p = 0.0344; HR = 0.97; 95% CI 0.94-0.99. Conclusions: Systemic inflammatory markers have demonstrated their diagnostic and predictive value in patients with neuroendocrine tumors of the gastrointestinal tract.