Eight examples of the Wilson-Mikity Syndrome are reported, 2 of whom died at 127 and 145 days of age respectively. The radiology of the condition is relatively specific and gives strong grounds for suspicion of the diagnosis when taken in the clinical context. Gross, histological, and electron microscopic examinations reveal areas of normality, emphysema, and immaturity. The electron microscope additionally shows areas where the alveolar wall thickening is due to the presence of smooth muscle, reticulin, and collagen. The most likely etiology is a disorder in postnatal alveolar development related to premature birth and functional use of an immature lung. There are changes in lung mechanics with decreased compliance and increased non-elastic resistance to airflow mainly in expiration. The mechanical work of breathing is increased. There is a respiratory acidosis in spite of an increased minute volume. Arterial oxygen desaturation is common, increased by crying and only partially relieved by oxygen. Large R → L shunts have been demonstrated which are probably intrapulmonary.