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The aim of this study was to examine the volume-pressure (V-P) characteristics of isolated upper airways in normal subjects and patients with obstructive sleep apnoea (OSA) and to ascertain whether an increase in upper airway muscle activity affects these characteristics. We studied upper airway pressure changes during volume changes by inflation and deflation of air volumes of 5, 10, 15 and 20 mL without and with submental electrical stimulation, during voluntary closing of the glottis, in seven normal subjects and 13 OSA patients. Volume-pressure properties of the upper airway were assessed by elastance (Euaw) which was obtained from the slope of the regression line of the V-P relationships. Euaw in OSA patients was 0.52 +/- 0.08 cmH2O/mL, which was greater than in normal subjects (0.26 +/- 0.06 cmH2O/mL). Submental stimulation increased Euaw in both OSA patients and normal subjects (0.70 +/- 0.11 cmH2O/mL and 0.41 +/- 0.11 cmH2O/mL, respectively). These results suggest that upper airways of OSA patients during wakefulness are less collapsible than those of normal subjects, and that, in both groups, submental stimulation may stiffen the upper airway.