Two patients are reported with a chondroid form of osteosarcoma, whose structure was also analyzed submicroscopically. The clinical and radiologic picture and the rapid lethal progress of the disease indicated an osteosarcoma, while both histologic and electron-microscopic investigation suggested the prevalence of malignant chondroid. Progress in the treatment of osteosarcoma makes it essential for diagnostic, therapeutic and prognostic reasons to reintroduce subclassification of these tumors in an attempt to correlate their morphological picture with their biologic properties. According to our experience to date, it seems that the cartilaginous component in no way ameliorates the rapid malignant penetration of the osteosarcoma. It is essential to distinguish these tumors from the usually much less malignant chondrosarcomas and to place the chondroblastic osteogenic sarcoma among in spite of the clear predominance of the malignant chondroid, and thus also introduce highly radical and combined therapy.