The objectives of our study were to demonstrate the patterns and sites of the upper airway (UA) collapse in obstructive sleep apnea/hypopnea syndrome (OSAHS) patients, utilizing the sleep endoscopy technique, and to describe the technique and summarize our experience in a large series of patients. UA findings during sleep endoscopy with midazolam were examined prospectively in 55 surgical candidates with OSAHS. The uvulopalantine was the most common site of obstruction (89%), followed by the tongue base, hypopharynx and larynx (33% each), and nose (21%); 72% of the patients had multiple obstructions. There was a significant correlation between the number of obstructions and the respiratory distress index (RDI). Laryngeal obstruction was typically supraglottic. Hypopharyngeal obstruction involved concentric UA narrowing. Our findings emphasize the considerable rate of laryngeal and hypopharyngeal obstructions in patients with OSAHS and suggest that their misdiagnosis may explain at least part of the high surgical failure rate of UPPP for OSAHS patients. The number of obstruction sites correlates with respiratory distress index. Sleep endoscopy is safe and simple to perform.