As part of a 4-year study of sudden unexpected infant deaths in St. Louis, we performed both a specialized death scene investigation and, in selected cases, a physiologic reconstruction of the death scene with a rabbit model. Those cases in which the infant was found face down with nose and mouth covered by bedding were chosen for the physiologic testing; our goal was to assess the potential for lethal rebreathing of expired air. The physiologic studies reconstructed the infants' premortem ventilatory environment by using the head from an infant mannequin positioned on the actual bedding on which an infant had died and a rabbit breathing through the mannequin's nares. The specialized scene investigation was carried out in 31 of 32 deaths from sudden infant death syndrome, diagnosed by the usual methods. Of 31 infants, eight died with their faces downward and covered by bedding, and the bedding was obtained for further study in seven of eight cases. The bedding had low resistance to airflow (6.25 to 22.6 cm H2O/L per second), and caused considerable rebreathing that was lethal to the rabbits in five of seven cases. We conclude that items of bedding in common use are capable of causing lethal rebreathing by prone-sleeping infants whose nose and mouth become covered; suffocation by rebreathing was the probable mechanism of death in a substantial number of these deaths that had been attributed to sudden infant death syndrome.