Context: We characterized linear growth in infants and children with X-linked hypophosphatemia (XLH).

Objective: Provide linear growth curves for children with XLH from birth to early adolescence.

Design: Data from four prior studies of XLH were pooled to construct growth curves. UX023-CL002 was an observational, retrospective chart review. Pre-treatment data were collected from three interventional trials: two phase 2 trials (UX023‑CL201, UX023‑CL205) and a phase 3 trial (UX023-CL301).

Setting: Medical centers with expertise in treating XLH.

Patients: Children with XLH, 1-14 years of age.

Intervention: None.

Main Outcome Measure: Height-for-age linear growth curves including values for the 5^th, 10^th, 25^th, 50^th, 75^th, 90^th, and 95^th percentiles for children with XLH compared to population norms. 

Results: 228 patients (132 girls, 96 boys) with 2,381 height measurements were included. Nearly all subjects (>99%) reported prior management with supplementation therapy. Compared to the CDC growth curves, boys at age 0.25, 0.50, 0.75, 1.0, and 2.0 years-old had median height percentiles of 46%, 37%, 26%, 18%, and 5%, respectively; for girls the median height percentiles were 52%, 37%, 25%, 18%, and 7%, respectively. Annual growth in children with XLH fell below that of healthy children near 1 year of age and progressively declined during early childhood, with all median height percentiles <8% between 2 and 12 years old.

Conclusion: Children with XLH show decreased height gain by 1 year of age and remain below population norms thereafter. These data will help evaluate therapeutic interventions on linear growth for pediatric XLH.