A case of progressive diaphyseal dysplasia with unusually severe deformities of the lower extremity is presented. It appears from reports of previous cases and from our experience that the muscle weakness is as important as the bone changes. When surgical correction of the deformity was carried out, with restoration of sound body mechanics, our patient was able to resume normal activity despite poor muscles. This disorder appears to be inheritable as a dominant trait, with irregular penetrance and a variable clinical expression. It is suggested that hereditary multiple diaphyseal sclerosis and progressive diaphyseal dysplasia are variants of the same genetic abnormality. A further effort should be directed toward controlled metabolic studies and a more comprehensive evaluation of the genetic abnormality. If care is taken to prevent or correct deformity, there is no reason to believe that the afflicted patient cannot lead a productive life.