The definition of obstructive sleep apnea (OSA) is an absence of air flow at the mouth and nose despite respiratory movement. In these patients, OSA does not occur during the awake state due to pharyngeal dilator muscles, but occurs due to loss of muscle tone during sleep. Moreover, sleep and anesthesia are related. Anesthesia and sedatives aggravate or precipitate OSA by the inhibiting neural and muscle activity and suppressing protective arousal responses. Therefore, the identification of OSA patients at greatest risk during the perioperative period is a major concern of anesthesiologists during preoperative visits. We experienced a case of acute upper airway obstruction in a child with unrecognized obstructive sleep apnea that occurred during anesthetic induction for adenotonsillectomy.