From the large database of patients enrolled in the Kabi Pharmacia International Growth Study (KIGS), 289 prepubertal patients with idiopathic growth hormone deficiency (GHD), treated for 2 years with growth hormone (GH) substitution therapy, were selected. A multiple regression analysis was performed to determine both the auxological factors characterizing the patients at the beginning of the first and second years on GH therapy and the respective treatment modalities relevant to the magnitude of the growth response. It was observed that during the first year on GH therapy the magnitude of the growth response was negatively correlated with chronological age and height SDS, and positively correlated with target height SDS, GH dose (IU/kg/week) and frequency of GH injections. During the second year the growth response was negatively correlated with chronological age and the first-year GH dose (IU/kg/week), and positively correlated with height velocity during the first year, GH dose (second year), and injection frequency (second year). The data suggest that the forces of 'catch-up'--auxologically entrenched within the distance between target height SDS and height SDS--no longer prevail during the second year of GH therapy. The inverse influence of the first-year GH dose in the two yearly phases of growth suggests that optimizing GH treatment must be attempted by analysing growth in response to GH over longer periods of time and considering that the growth process is influenced by interactive factors.