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Background. The availability of Ki-67 monoclonal antibody has opened new possibilities for an extensive analysis of cell kinetics in human neoplasms. Ki-67 antibody reveals a nuclear antigen that is expressed in proliferating but not in quiescent cells. Although the reliability of Ki-67 immunostaining has been evaluated in different tumor types, little information has been reported on bone neoplasms. Methods. Cell proliferation, as determined by Ki-67 expression, was measured by immunofluorescence on representative cytospins obtained from 205 patients with bone tumors. In each sample, the percentage of Ki-67-positive cells was quantified on at least 500 cells and expressed as Ki-67 labeling index (LI). Results. Ki-67 LI was lower in benign and low grade lesions as compared with high grade malignant lesions. A correlation between Ki-67 LI and histologic grade was observed in osteosarcoma and chondrosarcoma. In osteosarcoma, among the 43 primary lesions included in this study, 30 patients, all treated with the same regimen of chemotherapy and limb-salvage surgery, were selected to establish the prognostic significance of cell proliferation. The Ki-67 labeling was higher in patients with a good histologic response to chemotherapy. However, at a 24-month follow-up, a worse prognosis was associated with a higher proliferative activity, whereas no correspondence was found between the histologic response to preoperative chemotherapy and the disease free survival, suggesting that in high grade osteosarcoma the biologic aggressiveness expressed by high levels of Ki-67 LI may be clinically more relevant than the responsiveness to antineoplastic agents. Conclusions. In bone tumors, the level of Ki-67 expression correlates with the level of malignancy and is diagnostically and prognostically useful.