We studied upper airway structural changes induced by continuous positive airway pressure (CPAP) in obstructive sleep apnea syndrome (OSAS) patients using drug-induced sleep endoscopy (DISE). This prospective study was conducted at an academic secondary referral center. In total, 28 male OSAS patients (mean age 41.1 years) with only retropalatal level obstructions were enrolled. Measurements of the obstruction site were obtained in two steps: first a measurement was taken of the obstruction site in accordance with sleep apnea, then, a measurement was taken of the obstruction site in accordance with DISE-assisted CPAP titration, including quantitative changes in the occlusion site before and after CPAP in pixel format using an area calculation program. There was a tendency for persistent closing in cases of antero-posterior (AP) obstruction versus cases of lateral (Lat) obstruction in the CPAP titration. Lat obstructions showed a tendency to be wider than AP obstructions in the quantitative analysis. These results show that the pattern and degree of airway expansion after CPAP differ in accordance with the obstruction site.