Obstructive sleep apnea (OSA) is the most common type of sleep apnea and is characterized by repetitive episodes of airflow cessation during sleep as a result of the collapse of the pharyngeal airway (1). The prevalence of OSA syndrome has been estimated at 2% of the general population of children (2), 13–66% of obese children (3), and 4–9% of adults (2). Airway collapse during sleep is caused by a combination of anatomical and functional factors. Anatomical factors include inherently narrow upper airway resulting from 1) enlarged adenoids or tonsils, 2) a large soft palate, and 3) a large tongue. Functional factors include decrease in the activation of pharyngeal dilator muscles with sleep onset. Obesity contributes to a narrow airway and airway collapsibility (4), and the prevalence of obesity is on the rise. Despite breathing efforts, airway obstruction results in no gas exchange leading to oxygen desaturation and frequent arousals during sleep. Pathologic central sleep apnea (CSA) is relatively rare when compared with OSA and is characterized by absent respiratory effort as well as airflow cessation during sleep, accompanied by arousal and/or hypoxemia. Airway narrowing and/or closure occur during CSA (5). CSAs have been reported in obese children and adolescents with sleep-disordered breathing (3). Complex sleep apnea is a combination of OSA and CSA, and patients with complex sleep apnea exhibit CSAs even during a continuous positive airway pressure treatment of airway obstruction (6). Knowledge of obstruction sites with an imaging tool has been of great interest to researchers because direct inspection of the airway may facilitate treatment planning and may have the potential to improve treatment outcome (7–9). Imaging techniques including endoscopy, fluoroscopy, computed tomography, and magnetic resonance imaging (MRI) have been applied to subjects with OSA, and each approach has its own unique limitations (10). MRI is noninvasive, involves no ionizing radiation, and has provided valuable insights into understanding upper airway anatomy and function in patients with OSA syndrome. In prior studies, static 3D MRI revealed narrowed airway volume in patients with OSA when compared with age-matched controls without OSA (11). Respiratory-gated dynamic 2D (12) and 3D (13,14) MRI have been proposed to measure the changes in the upper airway caliber in relation to the respiratory cycle during tidal breathing and compare them between subjects with and without OSA syndrome. Real-time MRI (i.e., MRI using an acquisition time per frame that is short relative to the dynamic process of interest) has been used to demonstrate airway collapse in OSA patients during sleep with (15,16) and without sedation (17–19), but has been limited to a 2D slice of axial or mid-sagittal orientation. Recently, Shin et al. (18,19) demonstrated visualization of airway collapse in adult patients with OSA using real-time interactive multislice MRI, in which slice prescriptions were made in axial, mid-sagittal, and coronal slices. The most common approach is 2D mid-sagittal MRI, which provides only partial coverage of a narrowed upper airway and is very sensitive to imperfect localization and to patient head motion. Similarly, 2D axial MRI can reveal temporal changes in airway cross-sectional area, but it is easy to miss the actual site of collapse. Real-time 3D MRI would be desirable, because it provides complete coverage of the airway and does not require prior knowledge of the location of collapse. In this work, we propose a real-time 3D MRI acquisition using a novel temporal view order and a reconstruction technique that provides high spatial and temporal resolution with the combined use of parallel imaging and constrained reconstruction. We demonstrate its effectiveness in visualizing marked reduction of the pharyngeal airway during inspiratory loading in volunteers and identifying locations of airway narrowing and/or collapse in pediatric subjects with sleep-disordered breathing during natural sleep.