OBJECTIVES: To determine whether a single polysomnographic night was a valid measure of obstructive sleep apnea syndrome (OSAS) in children with symptoms of sleep-disordered breathing. STUDY DESIGN: The night-to-night variability of respiratory and sleep parameters was measured prospectively in 30 snoring children aged 1.6 to 11.3 years (mean +/- SD, 4.1 +/- 2) by using 2 nocturnal polysomnograms performed 7 to 27 days apart (14 +/- 5 days). RESULTS: The mean of the respiratory variables including apnea index, apnea/hypopnea index, arterial oxygen saturation, and end-tidal partial pressure of carbon dioxide were not significantly different from night to night. Among the sleep parameters, there was no significant night-to-night difference in sleep efficiency, arousal index, percent rapid eye movement, or percent of slow wave sleep. Only the percentage of stage 2 was significantly different between the nights. The polysomnographic clinical diagnosis remained the same on both nights for all children, although the disease severity differed slightly in 2 patients. CONCLUSIONS: There is little clinically significant night-to-night variability in pediatric polysomnography, and no first-night effect. These data suggest that a single polysomnographic night is an adequate measure of the OSAS in children with symptoms of sleep-disordered breathing.