BACKGROUND: Obstructive sleep apnea syndrome (OSAS) is associated with cardiovascular morbidity and mortality, and increased sympathetic activity is considered to be a causative link in this association. Higher levels of sympathetic activity have been reported in children with OSAS. Sympathetic predominance is indicated on heart rate variability (HRV) analysis by increased heart rate (HR) and a higher ratio of low-frequency to high-frequency band power (LF/HF). Improvement in OSAS after adenotonsillectomy (AT) in children with OSAS could, therefore, be associated with reduced HR and reduced LF/HF. METHODS: Changes in HR and time and frequency components of HRV were retrospectively analyzed in 2-min epochs free of respiratory events during light, deep, and rapid-eye-movement (REM) sleep in children with OSAS who underwent polysomnography before and after AT. RESULTS: Eighteen children with OSAS, aged 4.9 +/- 2.4 years (mean +/- SD) were studied. After AT, the apnea-hypopnea index decreased from 31.9 +/- 24.8 events/h to 4.1 +/- 3.7 events/h. The HR decreased after AT in all stages of sleep (99.8 +/- 16.9 beats/min to 80.7 +/- 12.9 beats/min [light sleep]; 100.2 +/- 15.4 beats/min to 80.5 +/- 12.4 beats/min [deep sleep)]; and 106.9 +/- 16.4 beats/min to 87.0 +/- 12.1 beats/min [REM sleep]), as did the LF/HF (1.6 +/- 2.7 to 0.6 +/- 0.5 [light sleep]; 1.2 +/- 1.6 to 0.5 +/- 0.6 [deep sleep]; and 3.0 +/- 5.4 to 1.4 +/- 1.7 [REM sleep]). CONCLUSIONS: The proportion of sympathetic activity of the autonomic nervous system declines in children with OSAS after AT in association with improvement in sleep-disordered breathing.