To identify factors influencing the response to GH therapy, we used a multiple regression model to analyze data from 632 naive prepubertal children with GH deficiency (GHD). There were 523 children with idiopathic and 109 children with organic GHD. They were treated with the same preparation of biosynthetic methionyl GH (somatrem, Protropin) for at least 1 yr. In children with idiopathic GHD, six variables predicted 40% of the response to treatment. They were (listed in relative importance, all P < 0.0001): age, log maximum GH, weight adjusted for height, dosing schedule, dose, and midparental height. Three variables, pretreatment growth rate, log maximum GH, and age, predicted 20% of the GH response in children with organic GHD. When data for all children were analyzed using analysis of covariance, children with idiopathic GHD grew better than those with organic GHD (mean +/- SD, 9.2 +/- 2.4 vs. 8.8 +/- 2.6 cm/yr; P < 0.0001). The children (both organic and idiopathic GHD) who did not respond well to treatment were younger and thinner than those who did. Early diagnosis and initiation of therapy should be beneficial to ultimate height attainment. The best response to GH therapy should be in young children with severe idiopathic GHD who receive daily weight-adjusted doses. The use of GH daily in higher doses would be expected to be most beneficial in older children with acquired and/or less severe GHD or in children who are underweight for height.