PURPOSE The upper airway volume is central to the development and treatment of snoring and obstructive sleep apnea, and mandibular advancement devices (MADs) have increasingly been used as an effective alternative to continuous positive airway pressure for these 2 conditions. We investigated the changes in breathing patterns and upper airway volume parameters measured on cone-beam computed tomography (CBCT) scans of patients with and without the use of custom-made MADs. MATERIALS AND METHODS We performed a prospective study at the Department of Oral and Maxillofacial Surgery, Odense University Hospital, on consecutively treated patients. CBCT scans were performed with and without the MAD to measure the changes in the upper airway volume. The patients underwent diagnostic cardiorespiratory monitoring before and after 3 months of MAD therapy. Measurements with and without MAD were compared using Student's t test and the Wilcoxon signed rank test, and mixed-model analyses were performed adjusting for sleep apnea severity, type 2 diabetes, body mass index, gender, and age. RESULTS A total of 44 patients (31 men and 13 women, age 50 ± 13 years, body mass index 31 ± 5.6 kg/m(2)) completed the trial. MAD therapy was associated with an increase in the total upper airway volume from 22.9 ± 8.7 cm(3) before treatment to 26.7 ± 10.7 cm(3) after treatment (P < .001). MAD therapy reduced the apnea-hypopnea index (AHI) from 15.8 ± 17.4 events/hour before treatment to 6.2 ± 9.8 events/hour after treatment (P < .001). CONCLUSION The results of the present study indicate that MAD therapy appears to produce significant changes in the upper airway volume that correlate with a decrease in the AHI.