Objective. A prospective sonographic study of the normal thymus was performed to determine the size changes with age and to compare the results according to some clinical conditions, such as sex, breast versus formula feeding, and term or preterm status. Methods. One hundred fifty-one healthy infants underwent thymic sonography Maximal transverse and longitudinal dimensions and anteroposterior dimensions of the right and left lobes were measured, and thymic indices were calculated. Mean values of thymic measurements for each group (sex, age, breast or formula feeding, and term or preterm status) were determined. The results were statistically evaluated. Results. Thymic dimensions showed the maximal values at about 4 to 6 months and gradually decreased after 6 to 8 months. No significant differences were found between mean values of thymic dimensions according to the sex and formula- versus breast-fed groups. As expected, the term group had greater thymic size values than the preterm group, probably because of positive correlations between thymus size, birth weight, and height. Conclusions. The thymus is clearly and easily visualized on sonography in the 0- to 2-year age period. This may be useful for evaluating qualitative and quantitative properties of the thymus and determining size changes according to age in various clinical conditions.