OBJECTIVES: Our study focused on the RANKL (receptor activator of nuclear factor-κB ligand)/RANK/OPG (osteoprotegerin) axis and selected proinflammatory/immunoregulatory upstream mediators in the peripheral blood (PBL) and cerebrospinal fluid (CSF) of multiple sclerosis (MS) patients. METHODS: PBL and CSF were collected from healthy controls (n = 35) and MS patients at the clinical onset of the disease (n = 33). In addition, PBL samples were obtained from relapse-remitting (RR)-MS patients (n = 30). Patients were assessed by means of the expanded disability status scale (EDSS) and routine laboratory parameters. Soluble (s)RANKL and OPG were measured in the CSF and plasma ; gene expression was detected for RANKL, RANK, OPG, and selected cytokines/chemokines (interleukin [IL]-4, IL- 10, IL-17, CCL2, and CXCL12) in PBL mononuclear cells. RESULTS: The OPG level in the CSF was lower in MS patients at clinical onset than in controls. Moreover, the sRANKL/OPG ratio was higher in the CSF of MS patients at clinical onset and in the plasma of RR-MS patients than in controls. Gene expression of RANKL/RANK/OPG in PBL mononuclear cells was higher only in RR- MS patients. IL-4, CCL2, and CXCL12 were positively correlated and IL-10 was negatively correlated with RANKL/RANK expression. OPG was negatively correlated with EDSS and alkaline phosphatase level. CONCLUSION: Our study revealed that changes of RANKL/RANK/OPG axis are associated with MS, particularly the decreased OPG level in the CSF at disease onset. Therefore, these factors may serve as disease biomarkers and molecular targets of novel therapeutic approaches.