Hereditary multiple exostoses is a familial disturbance in the growth of cartilaginous bone tissue, most marked at the diaphyso-epiphyseal junction of the long bones. Until the report of Ehrenfried (1) in 1917, mention of this disease was relatively infrequent in the American literature, but since that time numerous cases have been reported (2). In a review of the American literature prior to his publication, Ehrenfried was able to find 71 cases, to which he added 12 cases of his own. His contribution was to bring this condition to the attention of the medical profession in this country. The nomenclature is confusing. In Britain, where the disease was popularized by Keith (6), it is referred to as “diaphyseal aclasia.” Ehrenfried and others in this country preferred “hereditary deforming chondrodysplasia,” a vague term which does not describe the actual findings. “Hereditary multiple exostoses,” as proposed by Jaffe (3), appears to give the most accurate description of the disorder and because of its simplicity seems to be a practical designation. Caffey (4), in his most recent book on x-ray diagnosis, refers to this condition as “chondromatosis” but gives “hereditary multiple exostoses” as a synonym. The use of the term “dyschondroplasia” in this connection is unfortunate, since, as originally used by OIlier, it referred only to cases of unilateral skeletal enchondromatosis. Jaffe, who based his paper on the study of 28 cases, feels that hereditary multiple exostoses is not as rare as the literature would lead us to believe. Undoubtedly many cases are not discovered, since there are no lethal factors involved and this disorder does not affect the fetus in its development. It is extremely unusual for a newborn infant to manifest any signs of the disease. Usually it appears in early childhood and is discovered only by accident. The incidence of hereditary multiple exostoses is variable, but most authors agree that it occurs more often in males. Ehrenfried gives the ratio of males to females as about 3 to 1. In the chapter on genetics in the Text book of Pediatrics by MitchellNelson (8), transmission through the female, though rare, is mentioned. Ehrenfried's analysis according to nationality, showed 60 per cent of the cases reported in the German literature, 27 per cent in the French, and 8 per cent in the Enghsh, leaving only 5 per cent to be accounted for in the literature of other countries. Cases in Negroes have been reported. The lesions of multiple exostoses are more readily demonstrated by x-rays than by clinical methods. The long bones are chiefly affected, especially of the lower extremities. One usually finds the lesions arranged bilaterally and symmetrically, but cases have been described in which one side of the body seems to be the site of predilection. The skull may occasionally contain some lesions because it is preformed in cartilage, but usually it is not involved.