Considerable interest has been recently aroused in regard to the so-called thymic shadow in children. Our interest has been centered particularly about the thymic shadow in the new-born infant. With this in mind, we studied 100 consecutive new-born infants at Mount Sinai Hospital to determine, if possible, the frequency with which these shadows were observed and also their association with the so-called cardinal symptoms. The thymus has been widely discussed since the time of Plater, who in 1662 described the first case of "thymicolymphatic death." Especially has interest been aroused during the past ten years. Nearly everything associated with respiratory difficulties has been attributed to enlargement of the thymus. Morse1 in a recent paper said, "It has become the fashion of physicians to attribute all disturbances of infants not definitely associated with rickets to the thymus. This is particularly true when the x-ray shows what apparently is an enlargement