In patients up to 19 years old, the scan surface area and average length of the thyroid were plotted as a function of age, height, weight and calculated body surface area. On an average, the right thyroid lobe was longer than the left. There was good correlation between the mean length of the thyroid gland and the patient's age or height. The correlation was slightly less satisfactory with the patient's calculated surface area (in part this may be because some patients were over- or underweight). The plot of thyroid length versus weight of the patient showed a distinct bend, as though approaching an asymptote. With the assumption that this might be approximated as a rectangular hyperbola, replotting was done in terms of the reciprocal of thyroid length as a function of the reciprocal of the patient's weight. There was a highly significant correlation. The bend in the curve of thyroid length versus weight was tentatively related to the increased body weight that occurs at about age 9 years. In these normal children, there was an excellent correlation between the scan surface area of the thyroid and mean thyroid length (δ = 0.84). Plots of the scan surface area of the thyroid versus age, patient height, patient weight of the patient's surface area, gave a less satisfactory result than corresponding plots based on the thyroid length. The formula for thyroid length versus age can be combined with the data of Kay and co-workers to yield an equation which describes thyroid weight in children as a function of thyroid scan length.