The frequent association of Paget's disease and osteogenic sarcoma in patients over 50 years of age has attracted the attention of other observers. Our interest was aroused because of three cases recently coming under our observation, and a careful study of the subject was undertaken. We would reiterate Codman's1 suggestion that such cases deserve a separate division in the Registry classification because of their different clinical features. It seems to us also that this group may be important in giving a clue to one of the etiologic factors of osteogenic sarcoma. We know of no instance of the development of a benign giant cell tumor or endothelial myeloma occurring in a bone that was the site of a preexisting Paget's disease. For a clear understanding of the problem, it has seemed advisable to summarize briefly some of the known facts concerning Paget's disease. DESCRIPTION OF OSTEITIS DEFORMANS Sir James