Heliox is a blend of helium and oxygen with a gas density less than that of air that decreases airway resistance (Raw) in patients ventilated for status asthmaticus. We tested whether breathing an 80:20 mixture of helium:oxygen would reduce pulsus paradoxus (PP) and increase peak expiratory flow (PEF) in patients presenting to the emergency room with an exacerbation of asthma. After receiving 30 min of beta-agonist aerosols and intravenously administered methylprednisolone, 27 patients whose PP remained greater than 15 mm Hg and whose PEF remained less than 250 L/min consented to breathe heliox or room air for 15 min. PP decreased and PEF increased with time in control patients, indicating a time-related effect of routine bronchodilator therapy (p < 0.05). PP decreased in 15 of 16 patients during heliox, and the change with heliox was significantly greater than that during air breathing (p < 0.01). PEF measured with a Wright's peak flow meter calibrated for heliox increased in all patients breathing heliox. Again, the increase in PEF during heliox breathing was significantly greater than the corresponding change in control patients breathing air (p < 0.001). To the extent that PP reflects the inspiratory fall in pleural pressure, this reduction in PP indicates a substantial reduction in inspiratory Raw when the less dense gas is inspired through narrowed bronchi having turbulent flow regimes. The 35% increase in PEF while breathing heliox signals a similar reduction in expiratory Raw, which might diminish the hyperinflation often observed during an exacerbation of asthma.(ABSTRACT TRUNCATED AT 250 WORDS)