Recent epidemiological evidence has demonstrated an unequivocal decline in the incidence of sudden infant death syndrome (SIDS). Although this decline in SIDS is correlated with changes in infant sleeping position from prone to supine, the physiologic mechanism underlying this decline remains unknown. A better understanding of physiologic differences between the prone and supine sleeping positions may provide further insight into SIDS pathogenesis. The diaphragm is the main muscle comprising the inspiratory pump and it can be assessed noninvasively by ultrasound. The strength of diaphragmatic contraction is a function of its thickness. To examine whether supine position offers any mechanical advantage by increasing the diaphragm's strength of contraction, we measured diaphragm thickness using ultrasound (7.5Mz) in 16 healthy term infants [birth weight 3275 ±200g (mean ±SE), study weight 3180 ±200g, gestational age 38.8 ±0.3 wks, postnatal age 3 ± 1d, 8 males and 8 females]. Diaphragm's thickness was measured at the zone of apposition between the diaphragm and the rib cage in right mid axillary line. Other physiological parameters [heart rate (HR), respiratory rate (RR), transcutaneous oxygen saturation (SaO2)] were recorded continuously by an electronic monitor (Datascope, Passport). During quiet sleep and 1 hr postprandial infants were randomly assigned first to supine or prone position. Each infant was studied in both positions. All parameters were measured both at functional residual capacity (FRC) and at end inspiration (EI). We found that in prone position compared to supine position the diaphragm's thickness increased significantly both at FRC (2.27±0.1 vs 1.97 ±0.08mm; p=0.007) and at EI (2.91 ±0.22 vs 2.36 ±0.1mm; p=0.003). HR, RR, SaO2 were not significantly different in either position at FRC and at EI. From these data we conclude that in healthy term infants the diaphragm may be in a more advantageous position in regards to its strength when the baby is in the prone position compared to the supine position. Therefore, the reduction in SIDS associated with supine position must be attributed to some other, as yet unknown, mechanism.