Objective: To investigate the pre-and postoperative pattern of upper airway obstruction in obstructive sleep apnea (OSA) patients treated by uvulopalatopharyngoplasty (UPPP).Background: The response rate to UPPP in unselected OSA patients is generally about 50%.Methods: Intraluminal pressure measurements during sleep were employed to analyze the pattern of upper airway obstruction before and after surgery.Results: Ten patients with mild to moderate OSA (respiratory disturbance index 19.7 (16.9-27.5) events/hr underwent a full night polysomnography before and 114 (6 1-138) days after UPPP. UPPP resulted in a significant improvement in subjective snoring and daytime sleepiness, but did not significantly alter the severity of sleep-disordered breathing. Preoperatively, the major site of obstruction was located at the oropharynx in nine patients, seven of them had additional minor obstruction sites outside the oropharynx. Complete relief of upper airway obstruction was only observed in those two patients with collapse confined to the oropharynx.Conclusions: In unselected OSA patients, UPPP improved subjective snoring and daytime sleepiness but did not result in a significant improvement in RDI or sleep architecture. Our results emphasize the need for a pre-operative investigation of the upper airway during sleep to select patients with collapse confined to the oropharynx.