Three-dimensional multi-detector computed tomography (3D MDCT) analysis of the upper airway suggested that the lengthening of the upper airway in the absence of volumetric change may independently contribute to the severity of obstructive sleep apnea syndrome (OSAS) in adults. We sought to investigate the relationships among the length and volume of the upper airway to the severity of OSAS. A total of 73 subjects underwent 3D MDCT scanning and standard polysomnography. We measured the upper airway length (UAL), which was defined as the vertical distance from the hard palate to the hyoid in the mid-sagittal plane. We also used the height-adjusted UAL for analyses. Upper airway volume was measured using a 3D reconstruction of the cross-sectional area from the hard palate to the hyoid. The adjusted UAL showed a significant positive correlation with the apnea hypopnea index (AHI, r = 0.523, p < 0.000) and was a significant variable for predicting the AHI of OSAS patients in multiple stepwise regression analysis. Although the severe OSAS group had a greater adjusted UAL compared with that of other groups (p = 0.001), the volume of the upper airway did not show differences among groups.