Pneumomediastinum in infancy occurs more frequently than is generally appreciated. Itmay be found on roentgenographic examination in asymptomatic infants. The pathogenic mechanism has been reviewed. Clinical manifestation of respiratory difficulty and circulatory embarrassment are dependent upon the extent of pneumomediastinum and pneumothorax which may coexist. Six cases in children under 2 years of age are discussed in this paper. The diagnostic importance of displacement of thymus lobules by air in chest films is emphasized.