Seven cases of progressive diaphyseal dysplasia in three generations of one family were observed and data presented. The affected grandfather had six children, all but one of whom are described as documented cases, the other being probably affected. One of the grandchildren is documented as a definite case and three of thirteen other grandchildren probably were affected. The characteristic features of the dysplasia are the roentgenographic appearance of bilaterally symmetrical diaphyseal cortical thickening and autosomal dominant inheritance.