Abstract： Objective　To explore the stimuli triggering respiratory arousal. Methods　Twenty-five patients with excessive daytime sleepiness (EDS) were monitored by nocturnal polysomonography (PSG) and respiratory inductive plethysmography. Patients with obstructive apnea/hypopnea index (AHI) ≥5/h were diagnosed as obstructive sleep apnea syndrome (OSAS). Patients with Epworth Sleepiness Score ≥12, frequent inspiratory flow limitation related arousal during sleep and AHI < 5/h were diagnosed as upper airway resistance syndrome (UARS). Seven normal subjects were recruited as controls. Results　Patients with OSAS：AHI(32.8±19.1)/h, percentage of time spend when oxygen saturation lower than 90% (SLT90%) (11.3±16.5)%, arousal index (ArI) (35±17)/h, n=15. Patients with UARS：AHI(2.5±1.4)/h, SLT90% (0.1±0.1)%, ArI(30±16)/h, n=10. Normal subjects：AHI (5.9±4.4)/h, SLT90%(0.2±0.4)%, ArI(13±5)/h, n=7. Though the levels of AHI and SLT90% in patients with UARS were similar to those in normal subjects(H=2.92, P=0.87 or H=0.086, P=0.77), much lower than those in OSAS patients(H=12.3, P=0.000 5 and H=6.64, P=0.01), ArI in UARS was just as high as that in OSAS (H=0.49, P=0.48).Conclusion　Arousals from sleep in patients with obstructive sleep disordered breathing might be mainly caused by inspiratory flow limitation rather than by episodic apnea or desaturation.