Sibling correlations are reported for Fels skeletal ages from 2,622 mixed longitudinal hand-wrist radiographs of 285 boys and 276 girls. Because the observed correlations were irregular across age, they were smoothed by using kernel regression. The brother-sister correlations were generally lower than those for the other types of sibling pairs, but they were similar to the brother-brother correlations after 11 years. The sister-sister correlations after 12 years. The present data do not provide clear evidence of involvement of the X chromosome in the timing of skeletal maturation, but after 12 years the present data suggest such an involvement.