A mandibular advancement device effectively increases upper airway size in patients with obstructive sleep apnea. Three-dimensional volumetricupper airway changes, exact amounts of change in obstructed upper airways (smallest cross-sectional area), and anteroposterior and transverse changes with mandibular advancement devices are not clear. Thirty-seven patients with obstructive sleep apnea diagnosed with polysomnography were evaluated with the subjective Epworth sleepiness scale, percentage of oxygen saturation, and cone-beam computed tomography. Parameters were airway volume, smallest cross-section area, anteroposterior width, and transverse width of the upper airway. Patients received a mandibular advancement device, a Twin-block appliance that could be titrated with a jackscrew; posttreatment data were collected after 6months. A statistically significant improvement in mean oxygen saturation level was observed, from 87.97%±4.43% to 94.89%±1.54% (P<0.001). A significant mean increase in airway volume of 2360±2050mm3(P<0.001) was observed, from 12140±4773mm3 to 14500±5114.6mm3. A stastistically significant mean increase in the smallest cross section of 46.55±31.62mm2, from 81.95±55.23mm2 to 128.5±54.78mm2 was observed. Anteroposterior width increased significantly, from 4.99±1.65mm to 8.01±2.04mm. Transverse width increased significantly, from 27.67±8.52mm to 31.94±8.59mm. Mandibular advancement devices increased the mean upper pharyngeal airway volume in this cohort, and this increase in volume appeared to be related to increased oxygen saturation.