Enhanced osteolytic bone resorption is a hallmark of multiple myeloma. Several studies demonstrated an elevation of RANKL levels in the bone marrow microenvironment in multiple myeloma, but serum levels of soluble RANKL (sRANKL) revealed controversial results. One study reported elevated sRANKL in serum in myeloma patients, but using the same test, the levels in the majority of the patients were below the detection limit in the hand of other groups. Thus, a novel test was developed which measures both soluble and OPG-bound RANKL (total-RANKL, tRANKL). The objective of the present study was to investigate the clinical significance of circulating levels of tRANKL in monoclonal gammopathies of undetermined significance (MGUS) or multiple myeloma (MM). Serum levels of tRANKL were analyzed by ELISA in 128 individuals: 20 healthy donors, 20 with MGUS and 88 newly diagnosed patients with multiple myeloma. Multiple myeloma patients had significantly (P<0.001) higher serum tRANKL values (median 7.01 μmol/L, range < 2 – 57.6) than healthy controls (median < 2 μmol/L) and individuals with MGUS (median < 2 μmol/L). Serum tRANKL levels increased significantly (P<0.001) from Durie & Salmon stage I to stage III (median values: stage I = 2.98, stage II = 4.61, stage III = 10.45 μmol/L) und from ISS stage I to stage III (median values: stage I = 4.64, stage II = 7.98, stage III = 23.12 μmol/L). Furthermore the serum tRANKL concentrations were significantly elevated in MM Durie & Salmon stage I versus MGUS (P=0.01). Multiple myeloma patients with osteolytic bone lesions in conventional radiography had significantly higher tRANKL levels than those without bone lesions (median 8.34 vs. 4.02 μmol/L, P=0.01). In 32 patients with multiple myeloma in stages II and III, who received chemotherapy and a monthly bisphosphonate treatment with zoledronic acid or pamidronate additional serum samples after 3 months were available. There was a significant (P<0.001) decrease from pre- to post-treatment tRANKL concentrations after three months of treatment. Our study shows that serum tRANKL levels are detectable in the majority of myeloma patients using this novel ELISA test. Advanced disease stages and osteolytic bone lesions in multiple myeloma patients are associated with increased serum tRANKL levels. The decrease of tRANKL after 3 months of induction therapy and bisphosphonates suggest a positive effect of anti-myeloma treatment on RANK ligand expression in multiple myeloma patients.