Activity diary.

X-linked hypophosphataemia is a rare, genetic, lifelong disorder caused by phosphate-regulating endopeptidase homologue X-linked pathogenic variants and, if left untreated, is associated with a progressive accumulation of musculoskeletal manifestations. Burosumab is a fully human monoclonal antibody that targets circulating fibroblast growth factor 23 and directly inhibits its activity, thereby correcting the abnormal phosphate homoeostasis in people with X-linked hypophosphataemia (XLH). The efficacy and safety of burosumab has been demonstrated in a programme of clinical trials in children and adults. Few data describe the experience of adolescents with XLH receiving burosumab treatment before and after skeletal growth ends. This prospective, multicentre, mixed-methods study described the lived experience of adolescents with XLH treated with burosumab at the end of skeletal growth (NCT05181839). Using patient-reported outcomes, wearable devices, and interviews, we found low median symptom severity scores for pain (0.00), stiffness (0.00), and fatigue (1.75) on a 0–10 scale. Symptoms were usually triggered by physical activity but rarely interfered with daily life. Some adolescents reported emotional concerns related to XLH and treatment transition. These insights can inform patient support during transition to adult care.