1. Xanthomata of the synovial membrane of the joints are probably more common than has been hitherto supposed, as is exemplified by this group of nine cases collected over a brief period of time.

2. The preoperative diagnosis of joint xanthoma seems never to have been made. Obscure intermittent swelling of the knee joint—associated with pain and free fluid, occasional locking, and a movable tumor, usually medial to the patella—will frequently be found to be caused by xanthoma.

3. Aspiration of the joint with the recovery of dark or sanguineous fluid points to the presence of tumor. The demonstration of a large amount of cholesterol in the fluid is probably pathognomonic of xanthoma.

4. Xanthomata originate in chronic hemorrhagic villous arthritis.

5. The stroma cell is related to the reticulo-endothelial system. It is derived from the surface synovial-membrane mesothelium, which has reticulo-endothelial properties, and gives rise to: (a) the foam cell, (b) the giant cell, and (c) the pigmented cells found in xanthomatous tumors.

6. Joint xanthomata are related to some fundamental disturbance of lipoid metabolism. The blood cholesterol is frequently elevated.

7. Cholesterol formation probably takes place locally as a result of interstitial hemorrhage and is a decomposition product of hemoglobin. Its failure to be formed universally when interstitial hemorrhage occurs is partially explained by the systemic disturbance of lipoid metabolism which usually exists.

8. Joint xanthomata can be cured by radical excision. In the case of solitary tumors, local excision is sufficient, but, in the case of multiple or diffuse xanthomata, a subtotal or total synovectomy is usually necessary.

9. No instance of a benign giant-cell xanthoma undergoing malignant transformation was found.