OBJECTIVE Circulating IGF‐I and IGF binding protein‐3 (IGFBP‐3) levels both increase in puberty where growth velocity is high. The amount of free IGF‐I is dependent on the IGF‐I level and on the concentrations of the specific IGFBPs. Furthermore, IGFBP‐3 proteolysis regulates the bioavailability of IGF‐I. However, the concentration of free IGF‐I and possible IGFBP‐3 proteolytic activity in puberty has not previously been studied.SUBJECTS AND MEASUREMENTS We investigated serum levels of easily dissociable IGF‐I concentrations and ultrafiltrated free IGF‐I levels by specific assays in 60 healthy boys and in 5 boys with precocious puberty before and during GnRH agonist treatment. In addition, total serum IGF‐I, IGFBP‐1 and IGFBP‐3 levels as well as IGFBP‐3 protease activity were determined.RESULTS Free (dissociable and ultrafiltrated) IGF‐I concentrations were significantly higher in pubertal boys than in prepubertal children and correlated significantly with the molar ratio between IGF‐I and IGFBP‐3 (r=0.69,P<0.0001 andr=0.54,P=0.0008, respectively) and inversely with IGFBP‐1 (r=−0.47,P<0.0001 andr=−0.43,P=0.0003, respectively). Multiple regression analysis suggested that IGFBP‐3 level, and not IGFBP‐1, was the major determinant of the free IGF‐I serum level in normal boys. Free IGF‐I levels were elevated in boys with precocious puberty and decreased during GnRH treatment. IGFBP‐3 proteolysis was constant throughout puberty (mean 20%).CONCLUSIONS We conclude that easily dissociable and ultrafiltrated free IGF‐I serum levels are increased in boys with normal and precocious puberty and suggest that the increased free IGF‐I serum concentration in puberty primarily reflects changes in total concentrations of IGF‐I and IGFBPs secondary to increased GH secretion