Sudden unexpected death (SUD) occurs annually in the United States in more than 15,000 infants less than 6 months old—the greatest mortality during the first year after the neonatal period. These deaths are not only unexpected but mostly unexplained, and most of the hypotheses proposed for causation have been disproved. A previous communication reported 28 instances among 18,000 infants who were under regular supervision; additional cases may simply not have been retrieved. Almost all of these were diagnosed at autopsy as interstitial pneumonia with findings indicating asphyxia. It is hypothesized that these deaths occur in those infants (30%±) who are unable to breathe through their mouths and who make violent spasmodic efforts to establish a nasal airway. The resemblance to neonatal deaths of infants with choanal atresia strongly supports this hypothesis.