Abstract Context Bone age (BA) assessment and prediction of adult height has not been well studied in children with X-linked hypophosphatemia (XLH). Objective To assess BA and its utility in height prediction in children with XLH. Design Retrospective, cross-sectional and longitudinal assessments of BA using two standard methods in children with XLH. Mean values were used to calculate predicted adult height (PAH), which was compared to final or near-final adult height (AdHt) in patients who were at or near the end of growth. Setting Academic medical center Patients 56 children with XLH Intervention None Main outcome measures Bone age, predicted adult height Results Initial radiographs demonstrated BA delay (chronologic age–BA) of 1.2±1.0 (mean±SD) years in males, and 0.4±1.0 years in females (greater delay in males, p<0.05). 58% of males and 21% of females were delayed 1-2 years; 11% of males and 9% of females were delayed more than 2 years. For 4 males with no prior orthopedic surgeries, mean AdHt was 171.2±5.3 cm; PAH was 176.3 (±11.7) cm using Bayley-Pinneau methods and 173.0±6.8 cm per Tanner-Whitehouse methods. For 15 females without prior orthopedic surgeries, AdHt was 155.9±5.2 cm; PAH was 156.0±6.8 cm (Bayley-Pinneau) or 161.6±4.2 (Tanner-Whitehouse, which differed from AdHt, p<0.005). Conclusion BA is delayed in children with XLH, but more strikingly in males. Height predictions were within a range typically used in healthy children (±2 inches). The Bayley-Pinneau method appears to modestly overestimate AdHt in males, whereas Tanner-Whitehouse overestimates AdHt in females.