Objective: To validate a mathematical model developed by Ranke et al. (J Clin Endocrinol Metab 1999;84:1174–7783) to predict the GH response during the first years of GH replacement therapy. Patients and Methods: 38 children with idiopathic GH deficiency (GHD) met all inclusion criteria for the prediction model, but the group differed in some characteristics from the cohort from which the model was derived. Results: Using the model for the 1st year including maximum GH after stimulation and the equation for the 6th year, the predicted value corresponded well with actual height gain. Differences were found when the growth response of the 1st year excluding maximum GH and that of the 2nd–5th year were calculated, resulting in a significant underestimation of actual height gain (–0.63 to –1.07 cm/year). Conclusion: The mathematical prediction model tended to underpredict the growth response to GH treatment in our patients with pronounced GHD. The severity of GHD seems to be an important parameter for the 1st year prediction.