Objective: To evaluate the feasibility of multi-slice spiral CT scan to localize upper airway stricture in patients with obstructive sleep apnea syndrome (OSAS) during drug-induced sleeping. Methods: One hundred and fourteen patients diagnosed as OSAS by polysomnography were included in the study.Multi-slice spiral CT scan covering upper airway was performed at the end of inspiration and clear upper airway images were obtained in waking.After injecting 5 mg of midazolam intravenously slowly in 109 patients,CT scan was performed at apnea and clear upper airway images were obtained in sleeping.Cross-section area and minimal diameter of airway were measured and the parameters were compared under those two states.Upper airway was displayed intuitionisticly by using post-processing techniques. Results: One hundred and nine patients with OSAS finished the examination with a success rate of 100;.Airway obstruction at retropalatal level was observed in 62 patients,among whom 26 were associated with airway obstruction at retroglossal level,27 with narrower airway at retroglossal level in sleeping compared with that in waking,and 9 with no significant change of the airway at retroglossal level after sleeping.Narrower airway at retropalatal level in sleeping compared with that in waking was observed in 40 patients,among whom 20 were associated with narrower airway at retroglossal level in sleeping compared with that in waking,10 with complete airway obstruction at retroglossal level in sleeping,and 7 with no significant change of the airway at both retropalatal and retroglossal levels before and after sleeping.Minimal mean cross-section area of airway at retropalatal level was (72.60±45.15)mm~2 in waking and (8.26±18.16)mm~2 in sleeping; and minimal mean cross-section area of airway at retroglossal level was (133.21±120.36)mm~2 in waking and (16.73±30.21)mm~2 in sleeping (P<0.01).Minimal mean diameter of airway at retropalatal level was (6.91±2.23) mm in waking and (1.18±2.14) mm in sleeping; and minimal mean diameter of airway at retroglossal level was (8.68±4.32) mm in waking and (1.68±2.22) mm in sleeping (P<0.01). Conclusion: Multi-slice spiral CT with post-processing techniques can display the shape of the upper airway in patients with OSAS in sleeping,and can localize the upper airway stricture and assess its range accurately.