Chondrosarcomas of the synovium. Chondrosarcoma of the synovium, either primary or secondary to synovial chondromatosis, is rare. Ten cases of synovial chondrosarcoma were studied (four from the Mayo Clinic files and six from the authors' consultation files). Two were considered primary. In five cases there was evidence of preexisting synovial chondromatosis, and in the remaining three, there was a suggestion of preexisting disease. Several histologic features were found that were helpful to diagnose malignancy. The most important ones were loss of the "clustering" growth pattern typical of synovial chondromatosis, myxoid change in the matrix, areas of necrosis, and spindling at the periphery of chondroid lobules. Pulmonary metastasis developed in five of nine patients; three of these patients died. 