Abstract Objectives GX-H9 is a long-acting form of recombinant human GH under clinical development for both adults and children with GHD. In this report, 24-month efficacy and safety of once weekly and every other week (EOW) administration of GX-H9 were evaluated, in addition to Genotropin® switch-ability to GX-H9 after 12-month of treatment. Methods Subjects were randomly assigned to receive either one of three doses of GX-H9 (0.8 mg/kg/week, 1.2 mg/kg/week or 2.4 mg/kg every other week) or 0.03 mg/kg/day of Genotropin®. Treatment duration is 24-month for all patients in GX-H9 arms while patients in Genotropin® arm were re-randomized to one of three doses of GX-H9 at the completion of the first 12-month of treatment. Doses of GX-H9 were adjusted throughout the treatment period whenever necessary, based on IGF-1 levels. Results Out of 56 randomized, 54 received either GX-H9 or Genotropin®. Fifty subjects completed the 12-month treatment period. Of 50, 45 subjects completed the next 12-month, comprising 33 patients from GX-H9 and 12 patients who switched from Genotropin®. First year/second year mean±SD annualized height velocity (aHV) for 0.8 mg/kg/week, 1.2 mg/kg/week or 2.4 mg/kg every other week of GX-H9 were 10.50±2.54/9.14±1.96, 11.76±1.96/9.88±1.92 and 11.03±2.92/9.72±1.90 cm/year, respectively. First year mean±SD aHV for Genotropin® was 9.14±3.09 cm/year. Patients switched to one of the three doses of GX-H9 in the second year showed comparable aHV in the second year (8.73±2.69/7.60±0.90/9.13±1.07 cm/year for 0.8 mg/kg/week, 1.2 mg/kg/week and 2.4 mg/kg/EOW GX-H9, respectively). No significant slow-down of the growth was observed in the second year from patients who received GX-H9 throughout and patients who switched from Genotropin®. Mean change in height SDS after 12 months/24 months of GX-H9 treatment throughout from baseline treatment improved continuously (+1.10/+1.61 and +1.31/+1.89 and +1.15/+1.69 for 0.8 mg/kg/week, 1.2 mg/kg/week and 2.4 mg/kg EOW GX-H9, respectively). First year mean change in height SDS for Genotropin® was +0.92 SDS, and showed comparable improvement in height SDS after switching to GX-H9 weekly arms (+0.76 and +0.79 SDS for 0.8 mg/kg/week and 1.2 mg/kg/week, respectively). Most treatment-emergent adverse events were evaluated as unrelated to the study drug and were mild or moderate in severity. No new safety concerns were observed throughout 24 months of long-term GX-H9 treatment or after switching to GX-H9 from Genotropin®.Conclusions Growth response and safety profile of GX-H9 in children with GHD is comparable to those of daily GH, achieving robust growth rates after 24-month treatment. Subjects switched from Genotropin® in the second year, also showed substantial catch-up growth indicated by improvement in height SDS. GX-H9 has a unique potential to be a convenient long-term GH providing not only weekly but also twice-monthly treatment.