<h2>Summary</h2> An adequate anatomic cause ofdeath was demonstrated in 107 of 137 fetuses and infants who died during the neonatal period; anatomic causes of death were demonstrable in eighty-seven (85 per cent) of the 102 liveborn infants and in twenty of the thirty-five stillborn fetuses. Intrauterine anoxia, as evidencedby aspiration of excessive amounts of the contents of the amniotic sac, was the single leading cause of death, accounting for 16 per cent of the fetal and neonatal deaths. From the available evidence, it is concluded that repeated deep intrauterine respiratory movements with resultant aspiration of large amounts of amniotic debris are not a normal occurrence and that the presence of large amounts of amniotic debris in the lungs is evidence of fetal anoxia. Fetuses and infants dying as a result of fetal anoxia probably do so as the result of cerebral damage and not primarily as a result of the inhalation of amniotic debris. Intrapulmonary hyaline membranes were the leading cause of death in liveborn infants, accounting for the deaths of fifteen premature infants; such membranes were not observed in stillborn fetures. The pathogenesis of these intrapulmonary hyaline membranes is discussed. Although no conclusions are reached as to the nature of the material comprising the membranes, evidence is advanced to support the concept that vascular damage and increased capillary fragility may play a role in their origin. Major congenital anomalies wereresponsible for the deaths of fourteen fetuses and infants, and major or potentially lethal anomalies were present in three other patients. More than one anomaly was present in over 50 per cent of those fetuses and infants whose deaths were attributed to congenital anomalies. Minor congenital anomalies, some of which might assume clinical significance somewhat later in life, were often present in the absence of any major anomaly. Intraventricular hemorrhage wasthe single leading cause of death of previable premature liveborn infants. Although other factors play a part in their pathogenesis, prematurity is certainly the most important factor predisposing to intraventricular hemorrhage. Bronchopneumonia was consideredresponsible for the deaths of ten liveborn infants and was present in twenty-nine other nonsyphilitic fetuses and infants. Although some examples of intrauterine pneumonia may be secondary to aspiration of sterile, but irritating, contents of the amniotic sac, many are probably due to the aspiration of amniotic fluid which has been contaminated by bacteria. There was a significant increase in the incidence of pneumonia, presumably developing in utero, following prolonged labor or aspiration of excessive amounts of amniotic debris. Congenital syphilis was responsiblefor nine fetal and neonatal deaths. Splenomegaly, syphilitic osteochondritis, and extramedullary hematopoiesis were the most constant abnormalities found in these patients, followed by syphilitic pancreatitis, moderate hepatomegaly, and syphilitic inflammation of the lungs. Other less frequent findings included syphilitic involvement of the leptomeninges, bowel, adrenals, and hypophysis. Intracranial trauma, as manifested by tears of the dural septa with or without subdural hemorrhage, was responsible for seven fetal and neonatal deaths. Many tears of the dural septa probably heal spontaneously and give rise to no definite symptoms; the subdural hemorrhage which may accompany dural tears is the usual cause of death. Kernicterus was considered to be the primary cause of death in five liveborn infants and was present as a contributory cause in four other infants. The available evidence suggests that some of these infants represent examples of kernicterus occurring in nonerythroblastotic infants. In at least one-third of the fetuses and infants for whom maternal records were available and in whom no anatomic cause of death could be demonstrated, the maternal records suggested intrauterine anoxia as the cause of death. Prematurity, uncomplicated by otherdisorders, is seldom the sole cause of death. Careful post-mortem examinations will reveal, in addition to immaturity, a cause of death in the majority of premature infants dying during the neonatal period. Inflammatory lesions, many ofwhich are probably infectious in origin, play an important role as primary and contributory causes of death in the neonatal period; they were present in 52 per cent of the liveborn infants in this series. Histologic examination of multiple sections of the lungs is an essential part of any post-mortem examination of a fetus or newborn infant. In this series, histologic examination of the lungs alone would have indicated the cause of death in 35 per cent of the fetuses and infants. The pulmonary lesions encountered are frequently indicative of the cause of death, but can be interpreted properly only when correlated with the clinical record and with the findings of a complete post-mortem examination. Further studies are necessary for a better understanding of the pathogenesis and significance of many of the commonly observed pulmonary lesions.