A study has been made of 4117 consecutive births including 66 stillborn infants and 85 neonatal deaths. Autopsies were done on 22 stillborn infants and 71 infants dying during the neonatal period. From the autopsy study of neonatal deaths it was found that at least 15% of all live-born premature infants weighing between 1000 and 2000 gm. at birth had hyaline-like material in their lungs. The incidence of the pulmonary lesion was found to diminish progressively as the birth weight of the infants increased, so that it was practically non-existent in infants weighing over 3000 gm. at birth. It was found that 90% of all infants weighing over 1000 gm. at birth and dying in the first 48 hours of life had hyaline-like material in their lungs at autopsy providing their mothers' pregnancies and labors had been free of serious complications and providing that the infants themselves had no other clinically recognized disease or defect of a serious nature. No relation was found between the presence of the pulmonary lesion and the race, sex, order of birth of the infant or the age of the mother and length or type of labor. The lesion was not produced by the aspiration of food. The symptomatology of the infants dying with hyaline-like material in their lungs was characterized by marked respiratory difficulty, including retraction of the lower chest wall with inspiration. The symptoms were not pathognomonic, but few premature infants with similar symptoms survived. The pathogenesis of this pulmonary lesion remains unknown. The theory is advanced, based on the facts brought out in this study, that the hyaline-like material in the lung is the result of an etiologic agent that not only injures the lung of the fetus, but also causes the premature birth of the infant so injured.