To assess how patients with respiratory acidosis from acute severe asthma respond to helium-oxygen (heliox) mixtures.Consecutive case series.Urban community teaching hospital.Over a 2-year period, 12 asthmatics (mean age, 33.8 +/- 11.3 years) presented to the emergency department with acute respiratory acidosis (pH or = 45 mm Hg). All 12 patients were treated with heliox (60 to 70% helium/30 to 40% oxygen). Five patients received heliox through a ventilator and seven received heliox via face mask.Arterial blood gases (ABGs) were drawn immediately before and at a mean of 49.2 +/- 25.2 min after beginning heliox therapy. No therapeutic interventions were made between ABGs. For the entire group, the mean PaCO2 decreased from 57.9 to 47.5 mm Hg (p or = 15%) coupled with a rise in pH by > or = 0.05. Using this definition, there were eight responders (67%) and four nonresponders (33%). The responders had a shorter duration of symptoms (17.8 vs 78.0 h, p or = 96 h) duration of symptoms, and two eventually required intubation.Heliox can rapidly improve ventilation in patients presenting to an emergency department with acute severe asthma with respiratory acidosis and a short duration of symptoms.