Cancer-testis antigens expressed in osteosarcoma identified by gene microarray correlate with a poor patient prognosis. From 30% to 40% patients with osteosarcoma eventually experience medical failure; and few biomarkers of prognostic significance have been established. High-throughput methods like gene microarray analysis can help to identify molecular biomarkers that are useful for diagnosing osteosarcoma and targeting its treatment.Oligonucleotide microarrays were used to compare expression profiles of osteosarcoma cell lines and osteoblasts. Differentially expressed genes were confirmed by real-time polymerase chain reaction (PCR) analysis. Corresponding proteins were evaluated by flow cytometry and Western blot analysis in osteosarcoma cell lines and by immunohistochemistry in osteosarcoma tissues. The association between staining intensity and clinical outcome was analyzed further.Cancer-testis antigens, including melanoma antigen family A (MAGEA), chondrosarcoma-associated gene family, member 2 (CSAG2), and preferentially expressed antigen in melanoma (PRAME), were increased significantly in all osteosarcoma cell lines that were analyzed. Real-time PCR examinations indicated that cancer-testis antigen expression was frequent and coordinated in patients with osteosarcoma. The expression of MAGEA was confirmed by Western blot and flow cytometry analyses in osteosarcoma cell lines. Furthermore, immunohistochemical staining analysis suggested that MAGEA expression may be used to predict distant metastasis and poor survival. The adjusted relative risk for lung metastasis was 2.79 (95% confidence interval, 1.12-6.93; P = .028) for MAGEA-positive patients. Five-year survival rates for patients with and without MAGEA expression were 39.6% ± 8.4% and 80% ± 8.9%, respectively (log-rank test; P = .01).The combined use of an oligonucleotide microarray, a clinical database, and a tissue bank was useful for identifying molecular tumor markers. The frequent expression of MAGEA and other cancer-testis antigens in osteosarcoma indicates that they may be useful as diagnostic markers and targets of immunotherapy that warrant further investigation.Copyright © 2011 American Cancer Society.