A retrospective study of 88 cases of extraskeletal osteosarcoma revealed that this tumor affects adults almost exclusively, with a high incidence in patients older than 50 years, and is slightly more common in males (58%) than in female patients. The tumor occurred principally as a soft tissue mass in an extremity, with a predilection for the thighs (lower extremity, 46.6%; upper extremity, 20.5%) and the retroperitoneum (17%). Most were deep-seated and were firmly attached to the fascia, but occasionally they were freely movable and confined to the subcutis or dermis. Nearly all presented as an insidiously growing mass rarely causing pain or tenderness. The preoperative duration of symptoms ranged from 2 weeks to 25 years (median, 6 months). In 17 cases, it exceeded 2 years. A history of prior trauma to the site of the tumor was stated in 11 of the 88 cases (12.5%) and of radiation in five cases (5.7%). Microscopically, the tumors contained varying amounts of neoplastic osteoid and bone, sometimes together with islands of malignant-appearing cartilage. Like osteosarcoma of bone, extraskeletal osteosarcoma showed a striking variation in histologic appearance and focally resembled malignant fibrous histiocytoma, fibrosarcoma, and malignant schwannoma. Follow-up information was available for 65 patients. Eight (12.3%) patients were alive with no evidence of recurrence; 12 (18.5%) patients were alive with one or more recurrences; and five (7.6%) were alive with metastases. Twenty-eight of the tumors (43%) recurred and 39 (63%) metastasized. Forty (61.5%) of the patients with follow-up information had died, 36 from the tumor and four from miscellaneous causes. The prevailing sites of metastases were the lung, the regional lymph nodes, and bone.