Polyhistioma, a neoplasm whose basic cells are small and round like those of Ewing's sarcoma, but which differentiate into various mesenchymal structures, most often bone, is described. Those cases that produce cartilage have been called mesenchymal chondrosarcoma; the others therefore are mostly urecognized. The name "polyhistioma" is suggested in the hope that with this terminology more of these tumors will be diagnosed. Polyhistiomas are not excessively rare; they affect bone and soft tissue. Half the patients with polyhistiomas died within 2 years. Treatment has resulted in survival of over 20 years. Thirty-eight of the 144 patients with polyhistioma have survived 5 years or more following first definitive treatment. Thirty-seven cases of the disease are presented.