To investigate the influence of sleep position and sleep state on obstructive sleep apnea (OSA) severity in in children with Down syndrome (DS). DESIGN: Retrospective review. SETTING: Sleep disorders laboratory of a tertiary medical center. PARTICIPANTS: Children with Down syndrome and typically developing children matched for age, gender, apneahypopnea index (AHI), and year of polysomnogram. MEASUREMENTS AND RESULTS: Sleep variables from baseline polysomnography. Sensor-recorded position (supine, prone, lateral) was expressed as the percentage of total sleep time. The AHI was calculated in each sleep state (NREM, REM), position, and position-sleep state combination. Of 76 DS subjects (55 male) the median age and AHI were 4.6 years (range 0.2-17.8 years) and 7.4 events/h (range 0-133). In all subjects, AHI was higher in REM than NREM (p <0.05); however, the NREM AHI was higher in DS subjects than controls (p <0.05). Compared to controls, the percentage of prone sleep was greater in DS subjects (p <0.05), but the percentage of supine or non-supine (prone plus lateral) sleep was no different. For DS subjects alone, NREM AHI was higher in supine than non-supine sleep (p <0.05). CONCLUSION: In DS and non-DS children alike, respiratory events are predominantly REM related. However, when matched for OSA severity, children with DS have a higher NREM AHI, which is worse in the supine position, perhaps indicating a positional effect compounded by underlying hypotonia inherent to DS. These findings illustrate the clinical importance of NREM respiratory events in the DS population and implications for treatment options.