The American Academy of Pediatrics in 1992 recommended that term infants sleep nonprone due to the increased risk for Sudden Infant Death Syndrome(SIDS) with prone sleeping. However, there is no formal recommendation for preterm infants, nor has body position and its effect on sleep been evaluated. In 3 month old term infants, prone position results in more quiet sleep and total sleep time than supine sleeping. The increase in non-REM sleep and decrease in arousals seen with prone sleeping may help explain the increased vulnerability for SIDS in the first 4 months of life. We studied the effects of prone (P) versus supine (S) position during sleep at 36-37.5 weeks post-conceptional age (PCA) in six infants (mean gestational age of 30.5± 1.8 weeks; mean birth weight of 1342 ± 90g) recorded with polysomnography (PSG) for two consecutive daytime nap periods (randomized to P or S initial position). PSG was scored for Total Sleep Time (TST), Active Sleep (AS), Quiet Sleep (QS), and Indeterminate Sleep (IS). Apnea index was calculated as number of events ≥ 6 sec per hour of sleep. Periodic breathing (PB) was defined using criteria of Kelly and Shannon (1979), then compared between P and S as percentage of total sleep time. Using a two-tailed t test, there were no significant differences between the P and S groups. Our sleep studies of preterm infants 36-37.5 weeks PCA have not shown the differences in sleep organization previously shown in older term infants. Total sleep and quiet sleep times were similar for both P and S. The apnea index and% PB were higher in infants sleeping P but it was not significant. The increase in non-REM sleep and decrease in arousals which has been reported for P sleeping in older term infants may appear at a similar PCA in preterm infants, when the risk for SIDS is higher.