In order to detect the dysfunction of the upper airway muscles in awake patients with obstructive sleep apnea syndrome (OSAS), we examined the effect of a chemical stimulus (CO2) on flow-volume loops obtained during tidal breathing. Six flow-volume curves during forced maneuvers and six flow-volume tracings during rebreathing of CO2 were recorded in 20 patients with OSAS. All patients had numerous obstructive apneas (mean apnea index 61 +/- SD 33, percentage of obstructive apneas 76 +/- 29) associated with moderate to severe degrees of arterial oxygen desaturation. Of the 18 patients in whom at least six flow-volume loops could be obtained during CO2 rebreathing, 13 exhibited respiratory flutter (range: grade 1 to 6) during tidal breathing in the flow-volume tracing (RF VtCO2). The greater the degree of RF VtCO2 during wakefulness, the less was the lowest oxygen saturation (LSaO2) during sleep. In fact, all four patients with RF VtCO2 score 6 had LSaO2 below 30%.