This study was designed to investigate changes that occur in the upper airway of patients with obstructive sleep apnea who use a modified functional appliance. The experimental group included 10 men, all diagnosed with obstructive sleep apnea. Nuclear magnetic resonance imaging was carried out on each patient. One image was taken when the patient was in maximum habitual occlusion and another was taken while the patient wore the appliance. From the images gained, a three-dimensional reconstruction of the upper airway from the palatal plane was achieved. The images revealed statistically significant increases in the volume of the upper airway with the use of a modified functional appliance. Three-dimensional studies of the upper airway show greater detail and therefore seem likely to have more validity than two-dimensional studies.