Intrauterine respiratory movements were speculated from the ancient time, and numerous studies were carried out both in human and animal experiments. Results of these studies suggested that intrauterine respiratory movement exist regularly and there is free flow of amniotic fluid into fetal lung during the fetal life. Amniotic fluid contains desqumated epithelial cells, lanugo, and vernix caseosa. Thus, when the amniotic fluid is aspirated, these amniotic debris will also be aspirated. The effort of aspiration of amniotic debris were also studied by many investigators, and their report indicated that in normal gestation with normal delivery, aspiration of amniotic debris are minimal and gives no significant effect to neonatal infant. However, if the amniotic fluid contains excessive debris, by vigorous fetal movement due to intrauterine anoxia, a large amount of amniotic debris will be aspirate to cause disturbance in neonatal respiration. Demonstration of amniotic debris in the lung is carried out by the identification of desquamated epithelial cells or lipid which is present in vernix caseosa in alveolar lumen. Based on these observations, present study is planned to investigate the amount and distribution pattern of lipids in neonatally dead infant lung to correlate with aspiration of amniotic fluid to clinical data and coexisting pulmonary and systemic diseases. From the autopsy cases performed during January, 1973 and March, 1976, 26 cases of neonatal autopsies aged less than 10 days after the birth were subjected for the studies. From each case, sections of the lung were examined for the presence of amined for the presence of amniotic debris and pulmonary diseases after routine paraffin embedding and hematoxylin-eosin stain. For the demonstration of fat, frozen sections of lung tissue from each case were stained with oil red-O, and the amount and distribution of fat in the lung were recorded. The correlation of amount and distribution of fat with age of infants, gestation period, body weight, types of pulmonary and systemic diseaases were made and following results are obtained. RESULTS AND SUMMARY: 1. Fat is present frequently in the lungs of neonatally dead infants, and distributed widely in bronchioles, alveolar ducts, alveolar lumen, and alveolar wall. 2. As. The infant age is younger, less than 2 days, the larger amount and the wider distribution of the fat are noted. 3. The amount of fat was parallel to the amount of amniotic debris judged by desquamated epithelial cells in the alveolar lumina. 4. The most common associated diseases were congenital anomaly, prematurity, and respiratory distress syndrome.