Supplemental O2 is sometimes used to treat children with the obstructive sleep apnea syndrome (OSAS). However, its effects have not been studied. We therefore evaluated the use of supplemental O2 during sleep in children with OSAS. Oxygen and room air were delivered via nasal cannula at 1 L/min for 4 h each in a randomized, double-blind fashion. Twenty-three children were studied (mean age, 5 +/- 3 [SD] yr). Patients had a higher mean SaO2 and higher SaO2 nadir when breathing O2. There was no difference in the number (10.9 +/- 20.6/h on O2 versus 13.5 +/- 19.3 on room air) or duration of obstructive apneas. Although there was no overall change in end-tidal PCO2 when patients breathed O2, two children showed a significant increase. We conclude that breathing supplemental O2 during sleep in children with OSAS results in improved oxygenation and in most cases does not exacerbate sleep-disordered breathing. However, end-tidal PCO2 should be monitored in children with OSAS receiving O2 therapy. We speculate that supplemental O2 does not depress the ventilatory drive during sleep in most children with OSAS.