The underlying pathophysiological mechanism behind death in the sudden infant death syndrome (SIDS) is uncertain. Although infants dying of SIDS frequently have a postmortem examination performed, no specific diagnostic pathology in any organ system has been identified. Previous theories relating to the cause of death in SIDS have included increased lower airway closure. We examined the airway morphometry of 57 infants who died of SIDS and compared these findings with those obtained from 21 age-matched infants who had died of non-SIDS causes. Airway wall dimensions, epithelial thickness, and the area of smooth muscle within the airway wall were measured. Airways from infants who died of SIDS showed a significantly higher proportion of airway smooth muscle than control airways when corrected for age and sex (p < 0.01). There was no significant difference between the groups for wall thickness or epithelial thickness. Increased airway smooth muscle in infants who have died of SIDS may contribute to excessive airway narrowing, raising the possibility that the cause of death in this condition is related to abnormalities in lower airway function.