The aim of this study was to examine adaptive changes in the cross-sectional area of the upper airway during mandibular advancement and jaw opening and to explore whether these changes are directly related to morphologic features of the mandible. Fourteen nonapneic men participated in this study. A custom-made oral device was used to keep the mandible at 0% (F(0)V(0)), 50% (F(50)V(0)), 75% (F(75)V(0)), or 100% (F(100)V(0)) of maximum advancement, and at 50% (F(75)V(50)), 75% (F(75)V(75)), or 100% (F(75)V(100)) of maximum gape at F(75)V(0). Magnetic resonance imaging was used to investigate differences in the upper-airway cross-sectional area in these 7 positions. To eliminate excessive individual variation, changes in the mean (mean%) and minimum (min%) cross-sectional area of the upper airway were calculated as percentages of those at F(0)V(0). Cephalograms were taken to analyze morphologic features of the mandible. A dose-dependent change was seen when the mandible was advanced. The min% in F(100)V(0) was significantly greater than those in F(50)V(0) and F(75)V(0). The change in the upper airway size varied between subjects during jaw opening. At F(75)V(0), the min% of the whole upper airway was significantly associated with morphologic features of the mandible, such as the mandibular plane angle and lengths of the mandibular body and ramus.