This preliminary study investigated the effect of a mandibular advancement device on upper airway collapsibility in seven patients with obstructive sleep apnea. Overnight polysomnography and dynamic magnetic resonance imaging were performed at the retropalatal and retroglossal levels, and the apnea-hypopnea Index (AHI), anteroposterior and lateral distances, and airway volumes were recorded. The tests were repeated following a 3-month period of wearing a customized mandibular advancement device. A significant reduction in AHI (from 31 events per hour to 18.2 events per hour) and improvement in airway dimension at both the retropalatal and retroglossal levels were recorded, suggesting a baseline record for future studies with a larger patient sample.