BACKGROUND/AIMS The study aim was to develop and validate models for long-term prediction of growth in prepubertal children with idiopathic growth hormone deficiency (GHD) or Turner syndrome (TS) for optimal, cost-effective growth hormone (GH) therapy. METHODS Height was predicted by sequential application of annual prediction algorithms for height velocity in cohorts of GHD (n = 664) and TS (n = 607) as documented within KIGS (Pfizer International Growth Database). As height prediction models also require an estimate of weight, new algorithms for weight increase during the first to fourth prepubertal years on GH were developed. RESULTS When height was predicted from the start of GH treatment, the predicted and observed mean (SD) gain over 4 years was 30.4 (3.4) cm and 30.1 (4.9) cm, respectively, in GHD patients, and 27.2 (2.2) cm and 26.6 (3.5) cm, respectively, in TS patients. For all 4 years, gains of weight SD scores (SDS) were accurately described as a function of weight SDS and observed gain in height SDS (R(2) > 0.89). CONCLUSION In GHD and TS patients treated with GH, an accurate prepubertal long-term prediction of height development in groups is possible. Based on this, an optimal individual height outcome could be simulated.