We have analyzed longitudinal twin data by using a multivariate normal model to identify and quantify genetic effects over time on two main aspects of growth, height and skeletal maturity. The largest genetic contribution to the variance in both height and skeletal maturity coincided with the respective ages of peak growth velocity. The highest genetic covariance between these two traits coincided with the age of greatest acceleration of growth in height. These findings imply the existence of regulatory or structural genes that influence growth in both height and skeletal maturity. We also found sex differences in the rapport between velocities for height and skeletal maturity. These are consistent with a predominant role of estrogen in accelerating skeletal maturation in females, and the existence of additional mechanisms in males which may promote growth in height independently of the effects of gonadal sex steroids.