It is unclear how important age at onset of puberty is for adult stature. The growth effects of differences in timing of puberty have been studied on a bone age basis in 22 hypopituitary boys and on a chronological age basis in male subjects with early, normal or delayed onset of puberty. Very early onset of puberty results in short adult stature. This is because a marked reduction of prepubertal height gain is only partially compensated for by an increase in pubertal height gain. In contrast, very late onset of puberty determines no increase or a minor increase in adult stature. This results from a reduction in pubertal height gain, counterbalancing the increased prepubertal height gain. Differences in duration of growth are the major factor accounting for the different height gains observed in relation to timing of puberty, while mean growth rate shows only minor changes. The differences in duration of pubertal growth are paralleled by differences in the rate of bone maturation, which therefore do not account for differences in duration of puberty. It is concluded that, except in severely precocious puberty, manipulation of the timing of puberty is unlikely to affect final height to any great extent.