Flow oscillations on flow-volume loops (a "saw-tooth" pattern) have been reported as a manifestation of obstructive sleep apnea (OSA), but the specificity of this flow-volume loop pattern has not been determined. In a review of 2800 flow-volume loops performed in our pulmonary function laboratory, we identified flow oscillations in 40 patients, an incidence of 1.43%. Thirty-one of these 40 patients (77%) did not have OSA. In 16 of these 31 patients (52%), a structural disorder of the upper airway or neurologic disease involving the upper airway muscles was found. Flow oscillations were associated with physiological upper airway obstruction (UAO) in 14 of these 40 patients (35%), but were the only physiological evidence of upper airway dysfunction in the other 26 patients. Flow oscillations on flow-volume loops thus represent nonspecifically instability of the upper airway, caused by a variety of disorders, including OSA. Detection of this peculiar flow-volume loop configuration should lead to investigation of the upper airway and the surrounding musculature as it might be an early indicator of ongoing disorders eventually leading to UAO.