Purpose: The purpose of this study was to verify the usefulness of an oral appliance (OA) in patients with obstructive sleep apnea syndrome (OSAS), and to examine the relationship between the shift in mandibular position and fluctuations in the oropharyngeal space.<BR>Methods: Fourteen patients (5 females and 9 males; mean age 51.1 years) diagnosed with OSAS were evaluated by pulse oximetry before and after insertion of the OA. Similarly, lateral cephalometric radiographs for each patient were obtained while sitting and awake in an upright position, in the intercuspal position, with the OA and at maximum opening.<BR>Results: Using the OA, the oropharyngeal space increased significantly with the exception of the top of oropharynx. At maximum opening, the space significantly decreased in the lower part of the oropharynx, however the space in the upper part of the oropharynx was significantly increased. In the lower part of oropharynx using OA, there was a relationship between the anterior shift in mandibular position and the increase in oropharyngeal space. However, there was no significant relationship between the downward shift in mandibular position and the increase in oropharyngeal space. The oxygen desaturation index and cumulative percentage of time spent at saturations below 90 % during sleeping were significantly decreased in patients using OA.<BR>Conclusion: These results confirm that the OA is useful as a cure for patients with OSAS and it suggests that the excessive increase in occlusal vertical dimension using the OA is of no value in expanding the oropharyngeal space.