Recombinant human GH therapy to children with idiopathic short stature (ISS) increases growth velocity, but its effect on final height (FH) is still uncertain. The aim of this study was to investigate the effect of recombinant human GH on FH of patients with ISS who were treated according to two protocols in comparison to untreated historical controls. In study 1 (n = 24), all patients were treated with 14 IU (4.6 mg)/m(2) body surface x wk in the first year; thereafter the dosage was doubled if the growth response was insufficient. In study 2 (n = 34), patients were randomized into three arms: 18 IU (6 mg)/m(2) x wk; 27 IU (9 mg)/m(2) x wk; and 18 IU/m(2) x wk in the first year, followed by 27 IU/m(2) x wk thereafter. Observed or estimated FH was available for 53 patients. Thirty-four untreated controls from the same centers were available for comparison. Mean FH SD score in GH-treated children was -2.1, vs. -2.4 in controls (-2.4) (NS), but height SD score gain (1.3 vs. 0.7) and the difference between FH and predicted adult height (4.0 vs. 0.8 cm) were significantly greater. The growth response on an initial dosage of 27 IU/m(2) x wk (6.9 cm) was significantly better than on other regimens (2.8 cm). We conclude that a GH dosage of 27 IU (9 mg)/m(2) x wk to prepubertal children with ISS leads to a mean FH gain of approximately 7 cm, whereas regimens starting on lower dosages are less efficacious.