Although most patients with obstructive sleep apnea (OSA) are obese, it is not known how obesity contributes to airway collapse during sleep. The purpose of this study was to determine whether the volume of adipose tissue adjacent to the pharyngeal airway in humans is related to the degree of OSA. We studied 30 subjects, nine without OSA and 21 with OSA; two subjects were studied before and after weight loss. Adipose tissue was detected with magnetic resonance imaging using T1-weighted spin echo sequences. The volume of adipose tissue adjacent to the upper airway was determined by measuring the volume of all pixels in the intensity range of adipose tissue within the region bounded by the ramus of the mandible, the spine, the anterior border of the soft palate, and the hard palate. Polysomnography was performed with conventional techniques. All subjects had a collection of adipose tissue adjacent to the upper airway; the volume of this adipose tissue correlated with the number of apneas plus hypopneas per hour of sleep (r = 0.59, p < 0.001). Both patients who lost weight and had fewer apneas and hypopneas had a marked decrease in the pharyngeal adipose tissue volume. We conclude that adipose tissue is deposited adjacent to the pharyngeal airway in patients with OSA and that the volume of this tissue is related to the presence and degree of OSA.