Benign neoplasms composed of cartilage are frequently observed; malignant forms, however, are comparatively rare. Phemister (1) reported 10 cases of malignant cartilaginous neoplasms among 61 bone sarcomas which he studied. Geschickter and Copeland's (2) group of such tumors includes about 154 cases. Among the 3200 necropsies performed by the staff of the Department of Pathology, Yale University School of Medicine, between Sept. 21, 1917, and Aug. 27, 1934, there were 420 cases of malignant neoplastic disease with but a single chondrosarcoma. This and one other case recently observed in the New Haven Hospital are recorded here because they present some unusual morphologic and clinical features. Report of Cases Case I: Mr. W. DeJ., a thirty-four-year-old, white, married, grocery store manager, was admitted to the New Haven Hospital (Unit. No. 82766) on March 19, 1930, with a painful swelling just below his left knee. He stated that about two and a half years previously he struck his knee against a sugar barrel and that the pain and swelling which followed disabled him for about ten days. Since then there had been a swelling in the same location. About six months following the injury, roentgenograms were taken and x-ray therapy advised. This treatment, however, did not affect the swelling, which gradually increased and for about seven months prior to admission had caused a dull aching pain with weakness and lameness of the leg and pain in the knee on flexion. On examination, an ovoid, slightly raised, tender swelling was observed, beginning just below the patella, extending for about 10 cm. downward and 7 cm. across. The center appeared brown, slightly fluctuant, and was raised 1 cm. Dilated minute blood vessels were seen radiating into the distal and lateral borders. There was a definite fullness in the distal portion of the popliteal space, but no tumor mass could be palpated. Flexion of the knee was limited to about 80 degrees. Roentgenograms showed the diameter of the head of the tibia to be increased, the osseous structure to be replaced by a “soap-bubble” appearance, and a “trumpet-shaped” flaring of the cortex. A diagnosis of giant-cell tumor of the left tibia was made, and on March 21, 1930, an exploratory operation was performed (by Dr. Merrill K. Lindsay) and a portion of the growth removed for histologic examination. The specimen thus obtained consisted of several fragments of red-gray and gelatinous tissue measuring up to 1 cm. in diameter and containing some bony spicules. Microscopic preparations showed strands of cells with faintly staining cytoplasm and deep blue staining nuclei lying in a homogeneous pink or lavender staining substance. There were seen occasional narrow strands of fibrous connective tissue, some deep blue staining granular areas with no osseous structure, some freshly extravasated blood, and a few large mononuclear cells containing golden yellow pigment. The arrangement of the cells and their relation to the intercellular substance resembled the structure of embryonal cartilage (Fig. 2).