AbstractObjectivesTo assess whether body position during sleep differs among children with obstructive sleep apnea (OSAS) and controls, and to assess the effects of body position, obesity, and tonsillar size on respiratory disturbance. Four hundred and thirty consecutive children with polysomnographically demonstrated OSAS. And 185 age‐, gender‐, and ethnically matched children (Controls) were compared. The effect of sleep body position on respiratory disturbance was examined in OSAS, and also in relation to obesity and tonsillar size. Children with OSAS spent more time in the supine position than Controls (P < 0.01), with less time spent in the side position (P < 0.005). Obstructive apnea and hypopnea index (AHI) was similar in the three sleep‐related positions, but apnea index (AI) was significantly greater (4.6 ± 0.7/hr TST) in the supine position than in the side position (2.7 ± 0.3/hr TST; P < 0.001) or prone position (3.3 ± 0.5/hr TST; P < 0.01). Tonsillar size was not a contributing factor to positional differences in AI or AHI. Obese OSAS children had increased prone position (20.4 ± 2.0%TST vs. non‐obese: 10.9 ± 2.5%TST; P < 0.05), and displayed increased AHI and AI while supine. Non‐obese OSAS increased AHI in prone or side positions compared to supine (P < 0.01), with no significant differences in position‐dependent AI. Children with OSAS spend more time sleeping supine and less time on the side. Obese children with OSAS are more likely to sleep prone, suggesting that this position may promote upper airway patency in the presence of obesity. Although tonsillar size is not associated with positional differences in breathing, the presence or absence of obesity markedly modifies the effect of body position on respiratory disturbance. Pediatr Pulmonol. 2007; 42:374–379. © 2007 Wiley‐Liss, Inc.