In 1938 Lichtenstein coined the phrase "fibrous dysplasia of bone" for a pathologicoanatomical entity which he wished to combine with, on the one hand, or separate, on the other hand, from lesions described in the literature under various designations. Osteodystrophia fibrosa, juvenile Paget's disease, leontiasis ossea, osteitis fibrosa, von Recklinghausen's disease of bones, fibrocystic diseases, and osteitis fibrosa cystica are only a few of the terms involved in the discussion of this problem. According to Jaffe and Lichtenstein, the process of fibrous dysplasia commences as a fibrous-tissue proliferation within the medulla, replacing marrow, absorbing and replacing the cancellous and cortical bone. Metaplastic bone is laid down in this newly formed fibrous tissue, and occasionally islands of cartilage are developed and cysts may be formed. The most important feature of fibrous dysplasia is its skeletal distribution. The lesion is circumscribed and localized in the interior of the affected bone. It may