Magnetic resonance imaging (MRI) provides high-resolution images of the upper airway and is useful for assessing conditions associated with increased tissue water content. To determine whether nasal continuous positive airway pressure (CPAP) changes awake upper airway morphology in obstructive sleep apnea (OSA), we performed awake upper airway MRI scans on five male patients with moderate to severe OSA before and after 4 to 6 wk of nasal CPAP therapy. MRI scans were performed using spin echo pulse sequences to examine detailed anatomy and inversion recovery sequences to assess mucosal water content. Patients did not have nasal CPAP applied during the MRI scans. Axial and sagittal images were obtained, and tracings were made of the upper airway, tongue, and soft palate. Utilizing computer graphics, cross-sectional areas and volumes were calculated for each anatomic structure. A subjective grading system was used to assess upper airway mucosal water content. Pharyngeal volume and minimum pharyngeal cross-sectional area increased (p less than 0.05) and tongue volume decreased (p less than 0.01) following chronic nasal CPAP therapy. The increase in pharyngeal volume occurred mainly in the oropharynx (p less than 0.01). Upper airway mucosal water content decreased in the oropharynx (p less than 0.05). We conclude that chronic nasal CPAP therapy during sleep in patients with OSA produces changes in awake upper airway morphology. These changes may be due to resolution of upper airway edema. The upper airway of patients with OSA can be accurately and repeatedly assessed using MRI.