Background: Chronic inflammatory demyelinating polyneuropathy (CIDP) is characterized by significant clinical heterogeneity, and as such reliable biomarkers help to identify and measure disease activity. The purpose of this study was to investigate the role of lymphocyte/monocyte ratio (LMR) in CIDP diagnosis and disease activity classification. Method: This retrospective study involved 36 patients with CIDP and 36 healthy controls (HC). The CIDP patients were grouped into 23 active CIDP patients and 13 with long-lasting remission. Hematological parameters, C-reactive protein (CRP) and erythrocyte sedimentation rate(ESR), neutrophil to lymphocyte ratio (NLR), platelet to lymphocyte ratio (PLR) , and LMR were compared between the CIDP patients and HCs. Pearson or Spearman correlation analyses were used to evaluate the correlations between LMR and other indicators. Receiver operating characteristic (ROC) curves were used to determine the role of LMR in the diagnosis of CIDP.Results: The CIDP patients showed lower LMR compared to HCs (P<0.05), and active disease patients showed lower LMR levels compared to the remission group (P<0.05). Positive correlations were observed between LMR and Hb (Hemoglobin), RBC (Red blood cells), and A/G (Albumin/globulin ratio), whereas LMR was negatively correlated with NLR, PLR, RDW (Red cell distribution width) (P<0.05). ROC curves showed that the area under the curve (AUC) for LMR in the diagnosis of CIDP was 0.807 (95% CI=0.705-0.908) with a sensitivity and specificity of 77.78 and 75%, and the AUC (95% CI) for the combination of ESR, A/G and LMR was 0.885 (0.809- 0.961) with a sensitivity and specificity of 80.56% and 77.48% respectively. Conclusion: Our study suggests that LMR may be an important inflammatory marker for recognizing CIDP and assessing disease activity.