PROGRESSIVE diaphyseal dysplasia, so-named by Neuhauser, Schwachman, Wittenborg, and Cohen, in 1948,8is also known as Engelmann's disease.* Fairbank6believes that Cockayne4described the first case in 1920 and mentions the other few reports of the syndrome. Affected patients are weak and thin and are shorter than normal in stature. Their muscular development is poor; their gait, waddling. Some complain of pain in the legs. The diagnosis is made in the first decade of life on the basis of roentgenographic changes in the long bones. In the early stages symmetrical fusiform expansion occurs at the midshafts. Sometimes two separate diaphyseal foci of bone expansion are seen in the femora or in the tibiae, with the intervening shaft less intensely involved or almost totally spared. The outlines of the abnormal sections are usually smooth, but the bone pattern is amorphous in texture and of irregular density, with consequent