This study aims to investigate the expression of vascular endothelial growth factor (VEGF) and matrixmetalloproteinase-9 (MMP-9) in giant cell tumor of bone (GCT) and other osteolytic lesions in bone. By using semiquantitativeRT-PCR, we showed that three major isoforms of VEGF (121, 165 and 189) were expressed in GCTs,with isoform 121 being the most abundant. The expression levels of VEGF and MMP-9 mRNA were significantlyhigher in advanced GCTs (stage II/III) than in stage I GCTs.We further elucidated the cellular localization of VEGFand MMP-9 gene transcripts in GCT and other osteolytic lesions using an in situ hybridization assay. The resultsshowed that stromal tumor cells and osteoclast-like giant cells of GCT, fibrous stromal cells in anuerysmal bonecysts and fibrous dysplasia, and Langerhans-type giant cells as well as histocytes in eosinophillic granuloma, wereall strongly positive for VEGF and MMP-9 mRNA expression. In a prospective study, we performed VEGF andMMP-9 immuno-staining on paraffin sections of pathological tissues harvested from 48 patients (14 GCT, 10anuerysmal bone cysts, 10 eosinophillic granuloma, 4 fibrous dysplasia, 2 simple bone cyst, 2 osteomyelitis and 6patients with fractured femoral head as control). The results showed that the differences in VEGF and MMP-9expression between Stage I and other advanced Stages (II, III and recurrent) were highly significant (p <0.001),with advanced stages showing a higher mean expression. The difference between recurrent and Stage II and IIIlesions, was also statistically significant (p = 0.03 for VEGF, and p = 0.01 for MMP-9 expression), with recurrentlesions showing a higher mean expression of both VEGF and MMP-9. In conclusion, VEGF and MMP-9 expressionin osteolytic lesions of bone co-relates well with the extent of bone destruction and local recurrence. Their expression may therefore provide some prognostic indication of the possible aggressive behavior of the underlying pathology. ©2003 Elsevier Science Inc. All rights reserved.