Engelmann's disease, progressive diaphyseal dysplasia, is a rare disorder of bone of unknown etiology characterized clinically by muscle weakness, waddling gait, and crippling leg pain. The diagnosis is confirmed radiologically by sclerotic thickening of diaphyses of long bones and histologically by the absence of osteoclasts with definite evidence of decreased bone remodeling. Three affected children who were treated with corticosteroids showed remission of clinical symptoms. In one boy, tetracycline labeling was performed and biopsied fibular bone was studied histologically and by microradiographs prior to steroid administration and 4 months later. The administration of steroids in this patient was associated with increased bone resorption and histologic evidence of osteon formation with secondary remodeling. The mechanism through which steroids act, remains undefined, but based on changes found in bone histology, we believe that the clinical improvement in this patient is related to specific changes in bone physiology produced by the drug. In this disease, corticosteroids appeared to stimulate osteoclasis, decrease lamellar bone deposition, and thereby produce a more normal histologic picture in bone, Although Engelmann's disease is a rare disorder, it can produce crippling disability. Our studies suggest that steroids deserve further clinical trials in the treatment of this disease.