The first few minutes after birth are a critical time of adaptation of the newborn infant to extrauterine life. The adequacy of that adaptation has been evaluated by means of the summed Apgar score. In preterm infants, Apgar score may correlate less with adequacy of cardiopulmonary function because of developmental immaturity. Measurement of arterial oxygen saturation by means of pulse oximetry offers a physiologic, real time method of monitoring the progress of cardiopulmonary adaptation by which the clinician can evaluate the need for and success of resuscitative efforts. Four preterm infants are reported in whom pulse oximetry was useful in assessing the changes in oxygen saturation during resuscitation.