Forty-eight normal full-term Chinese babies (25 boys and 23 girls) were followed up every 2 mo in the first year and every 3 mo during the second year of life for anthropometric measurements. Blood samples were taken at birth and at 6, 10, 12, and 18 mo after birth for serum GH-binding protein, IGF-I, and IGF-binding protein 3 analysis. Onset of the childhood phase of growth in the infants was determined from the growth data plotted on Infancy-Childhood-Puberty growth charts. The serum GH-binding protein concentrations were low in cord blood but rose significantly at 6 mo, with slower rises in late infancy and early childhood. However, a significant rise in serum IGF-I and IGF-binding protein 3 levels was only observed from 10 mo of life onward. The change in IGF-I between birth and 6 mo was significantly correlated with length gain (r(2) = 0.35, p < 0.05) and body mass index gain (r(2) = 0.41, p < 0.01) during the same period. The 34 infants with onset of childhood phase of growth between 6 and 10 mo had a higher mean serum IGF-I value at 10 mo (8.8 +/- 5.8 nM versus 4.9 +/- 3.1 nM; p < 0.05) and higher length velocity between 10 and 12 mo (16.3 +/- 4.7 cm/y versus 8.8 +/- 4.3 cm/y; p < 0.001) compared with the 14 infants with a later onset after 10 mo of age. A significant correlation between a change in serum IGF-I and IGF-binding protein 3 levels was observed during the three 6-mo periods between birth and 18 mo, but a significant correlation between a change in serum GH-binding protein and a change in serum IGF-I or IGF-binding protein 3 levels was only seen between 12 and 18 mo of age. The multiple regression analysis (r(2) = 0.43, p = 0.0002) revealed that the change in serum GH-binding protein and IGF-I concentrations between 6 and 12 mo of age and the age of onset of childhood phase of growth could explain 43% of the length gain between 6 and 12 mo of age in our babies. The results of our study support the hypothesis that the onset of the childhood phase of growth is associated with the onset of significant GH action on growth.