Objectives: This study aims to evaluate the diagnostic andprognostic significance of neutrophil-to-lymphocyte ratio (NLR),platelet-to-lymphocyte ratio (PLR), and lymphocyte-to-monocyteratio (LMR) values in patients with osteosarcoma.Patients and methods: A total of 172 patients (111 males,61 females; mean age: 24.3±15.3 years; range, 7 to 82 years)diagnosed with osteosarcoma in our institution between January2002 and December 2018 were retrospectively analyzed. A totalof 165 healthy individuals (115 males, 50 females; mean age:20.2±9.2 years; range, 10 to 65 years) who did not have infectious,rheumatological or hematological diseases or any pathologicalfinding were assigned as the control group. The clinical, laboratory,and demographic findings of the patients were obtained fromhospital records. Pre-treatment NLR, PLR, and LMR values werecalculated in all patients. Diagnostic and prognostic values ofpre-treatment NLR, PLR and LMR were assessed using receiveroperating curve (ROC) analysis. The Kaplan-Meier method wasused for survival analysis.Results: For diagnostic approach, the highest significance inarea under the curve (AUC) values was obtained for NLR(AUC=0.763). The AUC for PLR and LMR was statisticallysignificant, while the statistical power was weak comparedto NLR (AUC=0.681 and 0.603). The NLR, PLR, and LMRwere found to be predictors of mortality. The cut-off value wasfound to be 3.28 for NLR, 128 for PLR, and 4.22 for LMR.The prognostic value of NLR for mortality was higher than(AUC=0.749) PLR (AUC=0.688) and LMR (AUC=0.609). TheNLR, PLR, and LMR were associated with overall survival (OS).There was a significant difference in the median OS time amongthe NLR, PLR, and LMR values (log-rank test order p<0.001,p=0.001, and p=0.004, respectively).Conclusion: Based on our study results, pre-treatment NLR,PLR and MLR have diagnostic and prognostic values inosteosarcoma.