A patient with hereditary progressive diaphyseal dysplasia with severe physical manifestations and bony changes is reported in detail. The diagnosis made at 18 months is the earliest reported. The progression of the physical and roentgenographic changes over a period of 3½ years are noted. In this patient, necrosis and atrophy of muscle were demonstrated in a biopsy. A thorough roentgenographic survey failed to reveal any changes in other members of the family.