Expression of CD31, Met/hepatocyte growth factor receptor and bone morphogenetic protein in bone metastasis of osteosarcoma. The mechanism of metastasis of osteosarcoma cells to other bones has not yet fully been clarified. The purpose of the present study was to examine whether various factors involve the formation of osteosarcoma metastatic foci in other bones. Immunohistochemically, CD31 expression in osteosarcoma with no bone metastasis and osteosarcoma with bone metastasis was noted in 10 and 75% of cases, respectively. Met/hepatocyte growth factor (HGF) receptor expression in osteosarcoma with no bone metastasis and osteosarcoma with bone metastasis was noted in 90 and 25% of cases, respectively. Bone morphogenetic protein (BMP) expression in osteosarcoma with no bone metastasis and osteosarcoma with bone metastasis was noted in 20 and 75% of cases, respectively. Metastasis of osteosarcoma cells to other bones was significantly correlated with expression of BMP and CD31 and with no expression of Met/HGF receptor protein in osteosarcoma cells. In contrast, expression of insulin-like growth factor receptor in osteosarcoma cells did not correlate significantly with bone metastasis. These results suggest that formation of metastatic foci of osteosarcoma cells in other bones is regulated by CD31, which is associated with migration between endothelial cells, by BMP, which can induce and activate various mesenchymal cells affecting bone formation, and by escape of effect by HGF, which promotes differentiation of osteosarcoma cells.