Matrix metalloproteinases participate in osteosarcoma invasion. Osteosarcoma (OS) is a highly malignant bone tumor and is the most frequent malignant bone tumor in children and adolescents. Metastases are the major cause of death, and patients with relapse have poor prognosis. Several solid tumors display enhanced expression of matrix metalloproteinases (MMPs), and recently MMP-inhibitors have entered clinical trials. A disturbance of the MMP system in favor of enhanced proteolytic activity may be suspected in OS because OS growth is accompanied by both enhanced local bone destruction and bone formation, two processes that are dependant on proteolytic enzymes. Thus, the aim of the present study was to evaluate the involvement of MMPs in a panel of human OS cell lines, xenografts and biopsies.Expression of MMPs and their endogenous inhibitors were studied by zymography and Northern blot analyses. In vitro invasion of OS cell lines and effects of MMP-inhibitors (Marimastat and doxycycline) were assessed in the transwell chamber assay.In vitro invasiveness was compared with gelatinase activity, and the most invasive cell line secreted the highest amounts of MMP-2 and MMP-9. Two different MMP-inhibitors significantly reduced OS cell invasion. The majority of the OS xenografts expressed both the inactive and active form of MMP-2 and in some cases also MMP-9. The biopsies from primary and metastatic OS also expressed MMP-2 mRNA. However, MMP-9 levels were higher in the biopsies than in the xenografts.The obtained results support the hypothesis that MMPs and their endogenous inhibitors participate in the invasive process of human OS.