Using spotted complementary DNA microarrays on glass slides, we have analyzed the gene expression patterns in human osteosarcomas, mesnchymal tumors of the bone. We determined the expression patterns of cell lines, tumors transplanted to immunodeficient mice and one patient sample. We compared the patterns of the same sample grown in vitro and in vivo, as well as one patient sample and its corresponding xenograft. The results indicate genes that may be important in osteosarcoma development or progression, as well as those expressed differentially in vivo and in vitro. We compared the expression patterns of osteosarcomas with those obtained from Ewing sarcoma cell lines.