OBJECTIVE Noonan syndrome (NS) is characterized by short stature, heart anomalies, developmental delays, dysmorphic features, cryptorchidism, and coagulation defects. Several studies reported the short-term effects of recombinant human growth hormone (rhGH) treatment on the improvement of height. This study was performed to evaluate the long-term efficacy of rhGH in children with NS in Korea. METHODS This study included 15 prepubertal NS children who received rhGH subcutaneously at a dose of 50-75 µg/kg/day for 6 days a week for at least >3 years. Preand posttreatment data, such as height, weight, bone age, insulin-like growth factor 1 (IGF-1), and IGF binding protein 3 (IGFBP-3) levels, were collected every 6 months. RESULTS Chronologic age and bone age at the start of treatment were 7.97±1.81 and 5.09±2.12 years, respectively. Height standard deviation score (SDS) was increased from -2.64±0.64 to -1.54±1.24 years after 3 years (P<0.001). Serum IGF-1 SDS levels were elevated from -1.28±1.03 to -0.10±0.94 (P<0.001). Height SDS was more increased in subjects without PTPN11 mutations compared to those with mutations after 3 years (P=0.012). However, the other parameters, including bone age, IGF-1 SDS, and IGFBP-3 SDS, were not significantly different between patients with and without PTPN11 mutations. CONCLUSIONS Although this study included a relatively small number of patients, long-term rhGH therapy in NS patients was safe and effective at improving height, growth velocity, and serum IGF-1 levels, in accordance with previous studies. However, the meticulous monitoring of potential adverse events is still needed because of high dose of rhGH and preexisting hyperactivity of RAS-MAPK pathway. Patients with PTPN11 mutations demonstrated a decreased response to rhGH therapy compared to those without mutations.