Primary malignant mesenchymoma of bone.

Primary malignant mesenchymoma of bone is a rare neoplasm consisting of two or more unrelated tissue elements other than a fibrosarcomatous component. To our knowledge, only six cases have been reported in the literature. A case of malignant mesenchymoma composed of rhabdomyosarcoma, chondrosarcoma, and osteosarcoma arising within the right acetabulum, resulting in the patient's death 4 months after presentation, is reported, and the literature is reviewed, with a discussion of the similarities to dedifferentiated chondrosarcoma.