Supplemental oxygen during sleep may be useful as a temporary palliative treatment in children with obstructive sleep apnea syndrome (OSAS) associated with significant hypoxemia. However, supplemental O2 may also blunt hypoxic ventilatory drive and worsen ventilation. To assess the safety of the use of supplemental O2 in children with OSAS, we studied 16 children ages 2-8 (mean: 4.28 +/- 2.88 yr) with OSAS secondary to adenotonsillar hypertrophy. Patients underwent two overnight polysomnograms within 1 mo, one on room air (RA) and one while receiving supplemental O2 via nasal cannula titrated by 1/4 lpm increments to achieve SpO2 > 95% during the first hour of sleep. Oxygenation measurements were significantly improved during supplemental O2 nights (average SpO2 increased from 89.5 +/- 4.8% on RA to 97.7 +/- 1.8% on supplemental O2 [p < 0.00001]) while alveolar ventilation remained unchanged (PETCO2 > 50 mm Hg: 3.6 +/- 8.9% total sleep time on RA and 3.3 +/- 6.3% total sleep time on supplemental O2 [p = NS]). Supplemental O2 significantly reduced hypopnea density, obstructive apnea index, and paradoxical breathing. The density and average duration of central apneas remained unchanged. In addition, supplemental O2 increased the percentage of REM sleep time and decreased the number of microarousals. We conclude that supplemental O2 might be a safe and beneficial temporary treatment in children with OSAS.