Although accidental suffocation has been suggested as a cause of sudden infant death syndrome in infants who are found prone with their faces straight down, the occurrence and effects on breathing of this position in living infants are unknown. We studied prone sleeping infants to see whether they will sleep with their faces straight down; whether they can spontaneously change from this position; the way that thermal stimuli or bedding softness influences this change; and whether rebreathing or airway obstruction occurs. We studied 11 healthy infants, aged 0.2 to 6 months, while they slept on soft and on firm bedding. Most infants slept facedown spontaneously or when turned to this position. All could turn their heads readily but slept facedown for variable periods. All infants put their faces straight down more often after cold than after warm stimuli. Obstructive apnea occurred only once, but rebreathing occurred in all subjects. When the infants were in the facedown position, inspired carbon dioxide was three times greater on soft than on hard bedding. End-tidal carbon dioxide partial pressure rose in all subjects while they were in the facedown position; in one infant, high end-tidal carbon dioxide partial pressure and desaturation occurred without signs of arousal. We conclude that infants sleeping facedown may occasionally have significant asphyxia; this sleep position may have a role in some infant deaths diagnosed as sudden infant death syndrome.