Study of Denosumab for Prevention of Skeletal Disease Progression in Children With Fibrous Dysplasia

* INCLUSION CRITERIA: In order to be eligible to participate in this study, an individual must meet all of the following criteria: * Confirmed diagnosis of fibrous dysplasia * Age 4 to 14 years * Concurrent enrollment in the companion Screening and Natural History protocol 98-D-0145 * Provision of signed and dated informed consent form * Stated willingness of guardian/Legally Authorized Representative (LAR) to comply with all study procedures and availability for the duration of the study * Ability of guardian/LAR to understand and the willingness to sign a written informed consent document * For females of reproductive potential: agreement to use highly effective contraception for during study participation. Highly effective contraception methods include: * Total abstinence. Periodic abstinence (e.g., calendar, ovulation, symptothermal, post-ovulation methods) and withdrawal are not acceptable methods of contraception. * Combination of the following (a+b or a+c, or b+c): * Use of oral, injected or implanted hormonal methods of contraception or other forms of hormonal contraception that have comparable efficacy (failure rate \<1%), for example hormone vaginal ring or transdermal hormone contraception * Placement of an intrauterine device (IUD) or intrauterine system (IUS) * Barrier methods of contraception: Condom or Occlusive cap (diaphragm or cervical/vault caps) with spermicidal foam/gel/film/cream/vaginal suppository * For males of reproductive potential: use of condoms or other methods to ensure effective contraception with partner * Minimum body weight of 12 kilograms EXCLUSION CRITERIA: An individual who meets any of the following criteria will be excluded from participation in this study: * Pregnancy or lactation * Known allergic reactions to denosumab * Prior history, or current evidence, of osteomyelitis/osteonecrosis of the jaw * Planned invasive dental procedure for the course of the study * Presence of non-healed dental or oral surgery * Orthopedic procedure performed less than 6-weeks prior to first day of the denosumab administration (Day 0) * Acute fracture less than 6-weeks prior to first day of the denosumab administration (Day 0) * Serum calcium or albumin-adjusted serum calcium below the normal range for the NIH laboratory (patients will be eligible for re-screening after a repletion period lasting up to 6 months) * 25-hydroxyvitamin D level than 20 ng/mL (patients will be eligible for re screening after a repletion period lasting up to 6 months) * Untreated or inadequately treated hypophosphatemia as determined by the principal investigator (patients will be eligible for re-screening after initiation or optimization of phosphorus replacement no longer than 6 months) * Inability to comply with a non-sedated 18F-NaF PET/CT (subjects will be eligible for re- screening after 6 months) * Use of another investigational agent within the last 3 months prior to the first day of the denosumab administration (Day 0) * Have any condition which in the opinion of the PI could present a concern for subject safety or difficulty with data interpretation.