The aim of this study was to evaluate the pharynx, soft palate, adenoid tissue, tongue, and hyoid bone when a mandibular repositioning appliance was used for managing patients with obstructive sleep apnea. Lateral cephalograms of 45 adult obstructive sleep apnea patients from the Lancaster Cleft Palate Clinic were taken without the appliance and some days later with the device in the mouth. This device was used during sleep for improving patients' respiration. Twenty-four cephalometric variables were evaluated. When the appliance was in the mouth, significant alterations (P < .001) were observed in the distances: (a) between anterior and posterior pharyngeal walls at the level of the second and third cervical vertebrae, respectively; (b) between the most superior point of the tongue and the maxillary plane; and (c) between the hyoid bone and the mandibular plane, ramus plane, cervical vertebrae tangent, mandibular symphysis, gonion, and third and fourth cervical vertebrae. The results of this study indicate that significant changes in pharyngeal space, hyoid bone, and tongue positions take place in obstructive sleep apnea patients when a mandibular repositioning device is used.