Seventy-eight children with idiopathic growth hormone deficiency treated with human growth hormone continuously for up to 5 years were studied; 58 were prepubertal at the start of treatment, and remained so throughout the treatment period, and 20 were showing pubertal signs at the start of treatment. Height velocity increased markedly in the prepubertal group over the first year of therapy, with a gradual decrease in the following years. The increased growth rate of the first treatment year continued in the pubertal group. Height standard deviation score for chronologic age increased significantly throughout all treatment years, but for bone age did not change in either group. The more frequent the treatment regimen, the better was the growth response; a dose effect independent of frequency was identified. Thus human growth hormone treatment cannot make up a deficit in height prognosis already present at diagnosis, but prevents further loss of stature, which is why early diagnosis is important.