OBJECTIVES: To investigate the adverse cardiac autonomic effects of sleep-disordered breathing (SDB) in a large population-based sample and a clinical sample of children. METHODS: Subjects included a population-based sample of 700 and a clinically diagnosed sample of 43 SDB children. SDB was defined based on an apnea hypopnea index (AHI) 1 during one night of polysomnography. Cardiac autonomic modulation was measured by heart rate variability (HRV) analysis of the beat-to-beat RR interval data collected during polysomnography. RESULTS: The mean (SD) age was 112 (21) months, with 49% male and 25% non-white. About 73.0% had AHI<1 (no SDB), 25.8% had 1-5 AHI (mild SDB), and 1.2% had 5 AHI (moderate SDB). Among individuals with moderate SDB in the population-based sample and the clinically diagnosed SDB patients, the mean (SE) of HRV-high frequency power (HF) was significantly lower compared to children without SDB [6.00 (0.32) and 6.24 (0.14), respectively, vs. 6.68 (0.04) ms(2), p<0.05 and p<0.01, respectively], whereas the low frequency power to high frequency power ratio (LF/HF) was significantly higher [1.62 (0.20) and 1.74 (0.09), respectively, vs. 0.99 (0.02), both p<0.01)]. CONCLUSIONS: SDB in healthy young children and in clinical patients is significantly associated with impaired cardiac autonomic modulation, i.e., sympathetic overflow and weaker parasympathetic modulation, which may contribute to increased risk of acute cardiac events in persons with SDB, even before reaching the "high risk age."