Background: Fibrous dysplasia (FD) is a disease that affects the bones. It causes bone lesions that can become weak and lead to fractures, deformity, and nerve injuries. FD bone lesions begin to develop soon after birth and grow during childhood. The lesions stop growing in adults but can still cause disability. Researchers want to find ways to stop the growth of FD bone lesions. Objective: To test a study drug (denosumab) in children with FD. Eligibility: Children aged 4 to 14 years with FD and who are also enrolled in the Screening and Natural History protocol (98-D-0145). Design: Participants will have a screening visit at the NIH clinic or by telehealth. Their medical history will be reviewed. Participants will stay overnight in the hospital 4 times in 76 weeks. Each stay will last 5 to 7 nights. Participants will also visit a local lab for blood and urine tests every 4 weeks during the study. Participants will receive denosumab once every 4 weeks for 48 weeks. The medication is given as a shot injected under the skin using a small needle. Some injections may be performed at home by a caregiver. The caregiver will receive training for this procedure. Participants will undergo many tests that may be repeated throughout the study. They will have a dental exam. They will have tests of their strength and ability to move freely. They will have x-rays and other scans to get pictures of their bones. Participants will be given another medicine that is administered through a needle in the arm over 30 minutes.