X-linked Hypophosphatemia Disease Monitoring Program

Inclusion Criteria: * Willing and able to provide informed consent or, in the case of patients under the age of 18 years (or 16 years, depending on the region), provide assent (if required) and informed consent by a legally authorized representative after the nature of the study has been explained, and prior to any research-related procedures. * Clinical diagnosis of XLH based on family history, OR confirmed PHEX mutation, OR biochemical profile consistent with XLH. * Willing and able to comply with the study visit schedule and study procedures. Exclusion Criteria: * Concurrent enrollment in an Ultragenyx-sponsored clinical trial is NOT permitted. * Serious medical or psychiatric comorbidity. * Less than one year of life expectancy.