The clinical records and histopathologic features in 26 cases of extraskeletal osteosarcoma (ESOS) diagnosed at M.D. Anderson Cancer Center (Houston) between 1950 and 1987 were reviewed. Presentation was usually that of an enlarging soft tissue mass. The thigh (11 cases), upper extremity/shoulder girdle (three cases), and retroperitoneum (three cases) were the most common anatomic sites. Tumor size ranged from 2.5 to 30 cm. The predominant histologic pattern was osteoblastic in four cases, chondroblastic in two, fibroblastic or pleomorphic malignant fibrous histiocytoma (MFH)-like in four, giant cell type MFH-like in one, and small cell in one. Various mixtures of these patterns were seen in the remaining 14 tumors. The telangiectatic pattern was not seen as the predominant component in any primary tumor but was observed as a minor component. Thirteen tumors recurred locally and 16 metastasized; five patients had distant metastases at presentation. The lungs, bone, and soft tissue were the most frequent metastatic sites. Sixteen patients died of disease at 2 to 54 months, one patient died of unrelated causes at 61 months, seven patients were alive with no evidence of disease (NED) at 30 to 122 months, and two patients were alive with disease at 28 and 54 months, respectively. Tumor size (less than 5 cm versus greater than or equal to 5 cm) was the main prognostic factor; all patients alive with NED for whom accurate tumor measurements were available (six of seven) had neoplasms measuring less than 5 cm that were amenable to complete surgical excision. Histologic pattern and other clinicopathologic features did not significantly affect outcome.