Objective: This study was carded out to determine whether bedding used by infants, who are at either high or low risk for sudden infant death syndrome (SIDS), differs in physical properties favoring rebreathing of exhaled gases. Study design: We compared softness and limitation of carbon dioxide dispersal by bedding, using a mechanical model. A questionnaire was used to describe sociodemographic risk factors and sleep practices; bedding was studied in homes with a model positioned where each infant was found sleeping that morning. Results: The groups differed with respect to five sociodemographic risk factors (p values all ≤ 0.0001). In addition, infants at higher risk were more likely to have been placed to sleep prone (46%, p = 0.02) by parents who were less likely to be aware of the risk associated with the prone position (62% aware, p = 0.005). Infants at higher risk had softer bedding (p < 0.0001, 54.1 ± 17.2 cm2 vs 33.7 ± 7.7 cm2 in contact with model), which caused more limitation of carbon dioxide dispersal (p = 0.008: CO2 retained, 0.60% ± 0.15% vs 0.34% ± 0.05%). Conclusions: A series of infants who are at high risk for SIDS because of sociodemographic factors more often sleep on bedding that has physical properties favoring rebreathing, and their parents are less often aware of the risk associated with prone sleeping.