The American Academy of Pediatrics has strongly endorsed a campaign to encourage the supine position as the preferred sleep position for healthy term infants. This recommendation was based on literature from New Zealand, the United Kingdom, and Norway. In the United States, the side position is still used by many parents, and recommended by as many as 55% of physicians. The effect of this sleep position on infants who present with apparent life threatening events(ALTEs) has not been investigated in this country. The purpose of this investigation was to compare the effect of prone versus side sleep positioning in infants admitted to a tertiary care children's hospital with ALTEs. Eligible infants included infants born at > 37 weeks gestational age, with a post-natal age between 5 days and 6 months who were admitted to duPont Hospital for Children with initial diagnoses of apnea, ALTE, choking episodes, or gastroesophageal reflux. Each infant was placed on a documented monitor for 12 hours, 6 hours in the prone position and 6 hours in the side position, with the starting position randomized. The number of alarms, episodes of apnea > 20 seconds, and bradycardia > 5 seconds were recorded. 40 infants were enrolled (mean±SD gest age =36±4 weeks; mean±SD postconceptional age = 44±6 wks, range 35-60). In the prone position the mean number of apneas was.27±.84, and the mean number of bradycardias was.44±1.45; the mean number of alarms was 1.5±2.2. In the side position the number of apnea and bradycardias were.39±1.12 and.49±1.94, respectively. The mean number of alarms in the side position was 1.65±4.22. The p value for apnea prone vs side was.90; the p value for bradycardia prone vs side was.90, and the p value for alarms prone vs side was.58. There were no significant differences between the two positions in each of these three variables. There was no effect of the order of sleep position on these variables. In summary, infants admitted to a tertiary care hospital, demonstrated no difference in apnea, bradycardia, or alarms in the prone or side sleep position. Since no demonstrable improvement was seen when the infants were in the side position, this reinforces the need in this country to encourage placement of all infants on their backs during sleep. We speculate the supine position may be preferable even for infants with gastroesophageal reflux, since their episodes of bradycardia and apnea were not improved by being placed on their side in this investigation.