The size of the thyroid was evaluated by sonography in 199 boys and 237 girls age one month to 17 years. The volume is 1.2/1.06 ml in boys/girls during the first month, 1.2/1.6 ml at the end of the first year of life, 1.7/2.4 ml at age four, 3.2/3.4 ml with eight years and up to the age of 12 years 5.7/5.7 ml. In juveniles older than 12 years the average volume is 8 ml, sex independently. However, the range appears to be rather wide (1.5-14.5 ml). The right lobe is usually larger than the left one. The rates of the length of the right lobe and the body length, of the total volume and the body weight, and of the total volume and the body surface area are almost independent of age for schoolchildren. There is no significant difference of the thyroid volume between the sexes.