The present study was designated to investigate causes of hGH treatment failure in 59 prepubertal children, with idiopathic or organic GH deficiency. Children with height gain of less than 6 cm during the first year of treatment were the oldest, those with tallest stature, and had a lower growth rate prior to treatment. Increase of growth rate was negatively correlated with pre-treatment growth rate. Poor results were chiefly observed in children with tumoral organic hypopituitarism or after irradiation of the brain. Finally hGH doses appeared to be similar when two groups with respectively poor and good results were compared, indicating a major role of endogenous factors.