The CT appearance of the normal and abnormal thymus and its age-related changes have been described. However, there is no information regarding the change in thymus CT attenuation values in children. The aim of the present study was to evaluate the normal CT attenuation of the thymus in infants and children and discover when the decline begins. CT attenuation values of the thymus were retrospectively evaluated in 152 children between ages of 1 day and up to 14 years. For each patient the mean value of the CT thymus attenuation was calculated and compared to CT attenuation of the chest wall and cardiac muscles. We also examined the correlation between thymic attenuation, gender, and disease. The statistical analysis used was multivariate linear regression. CT attenuation of the thymus declines with age, beginning only after the 1st year of life. The thymus is more hyperdense than the chest wall and cardiac muscles in infants less than 1 year. The thymus is denser in males than females. In malignant conditions, in infants less than 1 year, thymus CT attenuation is decreased. In older children thymus attenuation was similar to that for the chest wall and cardiac muscles. The decline in thymic CT attenuation with age is consistent with fatty infiltration of the gland. The measurements given in this report can serve as a basis for comparison to determine whether the thymus of a young child is normal or pathological.