To determine if excessive osteoclastic-mediated bone resorption (BR) is an early tumor-induced event in multiple myeloma (MM), BR was assessed at first presentation on quantitative bone biopsy in 87 individuals evaluated for monoclonal gammopathy of undetermined significance (MGUS) and reinterpreted according to the presenting features and subsequent follow-up evaluation. As a reference population, 48 patients with previously untreated overt MM were evaluated under similar conditions. The median level of BR was significantly higher in 48 overt MM versus 87 MGUS patients (12.2% v 5.1% [normal level, <6%], P <.01). Actually, 93% of overt MM patients had an excessive BR versus 45% of MGUS patients at presentation (P <.01) According to simple presenting parameters (> or <5% plasma cells within the bone marrow, presence or absence of mild anemia/neutropenia), 31 individuals were classified as low-risk MGUS, 32 high-risk MGUS, and 24 indolent MM. An excessive BR was observed in 16% of low-risk MGUS, 46% of high-risk MGUS (P <.01 v low-risk MGUS), 79% of indolent MM (P <.05 v high-risk MGUS), and 93% of overt MM patients. Of major interest, the level of BR in indolent MM (11.2%) was identical to that in overt MM (12.2%) but significantly higher than in both low-risk (4%, P <.01) and high-risk (5.6%, P <.01) MGUS. When considering the follow-up evaluation of MGUS patients, an excessive BR at presentation was observed in 52% of MGUS cases that turned out to be unstable or developed subsequent MM, but in only 4% of stable MGUS (P <.01). More precisely the level of BR of low-risk MGUS that either turned out to be unstable or that developed into MM was significantly higher at presentation than that of subsequent stable MGUS (4.4% v 2.9%, P <.05). The same difference was observed in both high-risk MGUS and indolent MM according to subsequent follow-up studies (8.1% v 3.4% and 11.7% v 6%, respectively, P <.05). Of major interest, the level of BR in 11 stable high-risk MGUS cases actually fulfilling the diagnostic criteria of smoldering MM was very low (3.4%) and similar to that in stable low-risk MGUS (2.9%). We conclude that a quantifiable excess of BR in MGUS is significantly associated with progression and thus is an early symptom of malignancy in these individuals.