Because premature infants have been shown to be at risk for hypoxia and bradycardia when positioned in standard car seats, this study was done to confirm this finding in a larger sample, to investigate convalescent term infants in the neonatal intensive care unit for respiratory compromise in car seats, and to determine the physiologic mechanism or mechanisms responsible. Extensive multichannel polygraph recordings were obtained and pulmonary function tests were performed on 50 convalescent infants from the neonatal intensive care unit before, during, and after placement in a Cosco-Peterson First Ride car seat. Mean total dynamic compliance, total pulmonary resistance, and work of breathing improved in the car seat. Thirty percent of premature infants experienced hypoxia, bradycardia, or both in a car seat; in this group, tidal volume was lower (p = 0.02). In 11 of 16 infants with abnormal findings, oxygen desaturation was temporally related to episodes of short and mixed apnea. No term convalescent infant experienced respiratory difficulty in a car seat regardless of primary diagnosis. We conclude that premature infants may have respiratory compromise of a multifactorial nature when in car seats. Further development of car seats is necessary if such respiratory problems are to be avoided.