We reviewed nine primary cardiac sarcomas with osteosarcomatous differentiation. The patients' ages ranged from 24 to 67 (mean 38 years). All tumors were surgical specimens from the left atrium; many were clinically diagnosed as atypical myxomas. In eight cases complete excisions were attempted, one requiring reconstruction with grafting; one tumor was biopsied only. Two tumors extended into the pulmonary veins. Three patients died within 2 weeks after the initial surgery from postoperative complications; five patients had metastatic disease or died from disease; and one patient was lost to follow-up. Metastatic sites included lungs, thyroid, and skin. In addition to osteosarcoma, four tumors showed chondroid differentiation, three had osteoclastic cells, four had a prominent spindle cell component, and one had myxoid areas. All tumors showed immunohistochemical positivity for vimentin; stains for cytokeratin and desmin were negative. S-100 positivity was demonstrated in chondrosarcomatous areas of one tumor. We conclude that most cardiac osteosarcomas are clinically mistaken for myxomas because of location in the left atrium. They are larger, tend to infiltrate, and are very aggressive neoplasms. Histologically a variety of patterns may be encountered in addition to the osteosarcoma.