It is usually believed that the size of the thymus increases up to the age of one year and perhaps it increases even until puberty. We performed a sonographic assessment of the thymic volume in nine healthy term infant 3-7 times within the first one-and-a half years of life. We found that the thymic size increased only during the first 6 months of life and then it decreased. Two children were atypical in that the size of the thymus increased again after 6 months of age. We suspect that the thymic size may be affected by infections and feeding modes and ongoing larger sonographic studies will hopefully reveal this.