The ultradistal (UD) radius is rich in trabecular bone and is easily measured by dual energy X-ray absorptiometry (DXA). UD radius areal bone mineral density (aBMD) may help identify trabecular bone deficits, but reference data are needed for research and clinical interpretation of this measure.We developed age-, sex-, and population ancestry-specific reference ranges for UD radius aBMD assessed by DXA and calculated Z-scores. We examined tracking of UD radius aBMD Z-scores over 6 years and determined associations between UD radius aBMD Z-scores and other bone measures by DXA and peripheral quantitative computed tomography (pQCT).Multicenter longitudinal study.2014 (922 males, 22% African American) children ages 5-19 years at enrollment who participated in the Bone Mineral Density in Childhood Study.UD radius aBMD.UD radius aBMD increased non-linearly with age (p<0.001) and tended to be greater in males vs. females (p=0.054). Age-, sex-, and ancestry-specific UD radius aBMD reference curves were constructed. UD radius aBMD Z-scores positively associated with Z-scores at other skeletal sites (r=0.54-0.64, all p<0.001) and pQCT measures of distal radius total volumetric BMD (r=0.68, p<0.001) and trabecular volumetric BMD (r=0.70, p<0.001), and was weakly associated with height Z-score (r=0.09, p=0.015). UD radius aBMD Z-scores tracked strongly over 6 years, regardless of pubertal stage (r=0.66-0.69; all p<0.05).UD radius aBMD Z-scores strongly associated with distal radius trabecular bone density, with marginal confounding by stature. These reference data may provide a valuable resource for bone health assessment in children.