Summary Recent studies suggest that multiple myeloma (MM) triggers osteoclastogenesis by disrupting the balance between the receptor activator of NF-kB ligand (RANKL) and osteoprotegerin (OPG), its natural antagonist. Determinations of bone marrow (BM) and serum OPG and sRANKL concentrations were performed in 133 MM patients and 42 healthy subjects by the ELISA method using Osteoprotegerin ELISA and sRANKL ELISA kits. MM patients had elevated serum levels of OPG compared with controls (p<0.0001) and OPG levels were higher in patients with renal failure and patients with hipercalcemia (p<0.001 and p=0.04, respectively). Serum OPG levels correlated with age, serum ?2-microglobulin, and BM OPG concentrations and did not correlate with the presence of osteolysis or with stage of disease. sRANKL serum levels in MM patients and in controls were not statistically different (p=0.42). In MM patients, serum OPG and sRANKL levels were similar at diagnosis and in the plateau phase of disease. There was a correlation between BM and serum sRANKL concentrations (p<0.001). Median values of the sRANKL/OPG ratio for BM and serum of MM patients were 0.14 and 0.11, respectively. The median value of the sRANKL/OPG ratio for the serum of controls was 0.11. In 20% of MM patients, serum OPG levels are elevated, and this may be a compensative reaction related to increased bone destruction. There is not statistically significant relationship between sRANKL serum and BM levels and the main clinical and laboratory parameters of the disease. Determination of BM and the serum sRANKL/OPG ratio seems to have no clinical value.