Serum levels of IGF-I were radioimmunoassayed after acid ethanol extraction in 1075 normal subjects from infants through young adults, and the normal range for each age was established. The mean value for infants which was relatively low increased gradually with age, and rose sharply after that reaching the peak levels at mid adolescence, then it decreased slowly to the young adult levels. Significantly higher mean values were observed in females at the age of 9, 10, 11 and 12 years. Each of 23 cases with pituitary dwarfism exhibited a lower concentration than the lower limit of the bone age matched normal range. All of the 59 normal variant short children except three showed normal values, but the values were distributed over the lower side of the range.