Fibrous dysplasia is a local developmental fibro-osseous aberration of the skeleton. The aetiology is not clear, but it does not appear to be hereditary. Increased production of Il-6 has been implicated as a causative factor. The disease occurs mainly in the first two decades of life, and both sexes are affected (Fig. 57.1). When there are polyostotic fibrous dysplasias of bone together with café au lait skin pigmentation and endocrine disorders, the condition is known as Albright syndrome or McCune-Albright syndrome (MAS) (Fig. 57.2). The underlying pathologic process is substitution of fibrous tissue for both bone marrow and bone with activation of osteoclasts. The aim of therapy is normalisation of the increased osteoclastic activity by administration of bisphosphonates.