This paper is a review of the literature dealing with xanthoma and of the cases observed at the Mayo Clinic and an attempt to arrive at certain definite conclusions by which this lesion can be properly classified. Owing to the histologic similarity of the synovial membrane of the tendon sheaths and that of the joints,1 it was felt that growths involving both these structures should be considered. Other types of solitary xanthoma occur in bone and in subcutaneous tissue, but they will not be discussed in this paper. An effort has also been made to refrain from any reference to the general subject of the xanthomatous diseases except when it is necessary as an aid in establishment of the true nature of these solitary tumors. HISTORICAL REVIEW The first reference in the literature to the group of conditions now listed under the general heading of xanthoma was by the