Six patients with obstructive sleep apnea were studied with our system of simultaneous video recording of the polysomnographic record and the endoscopic image from the pharyngeal airway. Preoperative and postoperative recordings were made during sleep in each patient. Tracheotomized patients recreated their preoperative laryngeal inlet obstruction and its immediate cessation by alternately opening and closing the tracheotomy tube. This demonstration coupled with the physical examination findings of "disproportionate anatomy," leads to the determination that the mechanism of obstructive sleep apnea is twofold: (1) an underlying CNS propensity to hypotonia of pharyngeal musculature during sleep and (2) either an isolated obstructive upper airway lesion or a combination of alterations in normal relationships within the upper airway that cause a passive narrowing of the upper airway. This combination of altered relationships is collectively referred to as "disproportionate anatomy."