Background: Based on the reported favourable effects of growth hormone (GH) treatment on growth and body composition in Prader-Labhart-Willi syndrome, we studied age dependency and the long-term effects on growth dynamics to elucidate the assumed hypothalamic GH deficiency. Methods: We examined 23 children treated with hGH (24 U/m2/week) during a median of 4 (range 1.5–5.5) years; group 1: 10 young underweight (age 0.3–4.1 years), group 2: 8 prepubertal overweight (age 3.7–9.5 years) and group 3: 5 pubertal overweight children (age 9.0–14.6 years). Results: After 4 years of therapy, height gain amounted to 1.8 SD; height (0.0 SD) and hand length (–0.2 SD) were normalised in the 2 prepubertal groups; in children above 6 years, height prediction approached parental target height. Weight for height rose in group 1 (to 0.64 SD) and decreased in group 2 (to 0.71 SD) to normal levels. Bone maturation of the pubertal children was too advanced to show a clear growth response to GH (height gain 0.42 SD). Even in this group, weight for height was reduced, but remained supernormal. Conclusion: Under exogenous GH, growth and body proportions are normalised in prepubertal children. With early institution of treatment, final height prediction reaches the parental target height range after 3 years. Such a growth-promoting effect of exogenous GH has so far only been described in children with GH deficiency.