One hundred and four larynges removed from infants registered as dying from Sudden Infant Death Syndrome (SIDS) and 20 control larynges from infants of matching ages, were examined histologically. Excessive amounts of subglottic submucosal glandular tissue had reduced the available airway by more than half in 35 per cent of the SIDS larynges within the age group two of four months. In 30 per cent of this group the airway reduction was more than 60 percent. It is suggested that hyperplasia of subglottic mucous glands is one cause of fatal hypoxia in sudden unexpected infant death.