Abstract. Using a protein-binding assay which measures mainly IGF I, serum levels of insulin-like growth factor (IGF) were determined in 263 children and adolescents of constitutionally variant stature but with normal GH secretion following provocative stimuli. A positive correlation was found between height age and the logarithm of IGF levels (r = 0.67, P < 0.001). Furthermore, a correlation was found in the tall and short subjects as a group between the ratio of IGF/normal IGF for age and between the ratio of growth rate/normal growth rate for age (r = 0.53, P < 0.001). Subjects were compared at the same stage of development. At a given bone age or stage of puberty, tall subjects had significantly higher IGF levels than short subjects (P < 0.005). After the completion of growth, IGF levels in both short and tall subjects stabilized within the normal range. Nevertheless, their mean levels remained significantly different (P < 0.001). Our results suggest that in normal children and adolescents, differences in IGF I secretion may be at least partially responsible for the individual differences in growth.