Thirteen patients with an acute exacerbation of asthma and pre-treatment FEV1 between 20%-60% of predicted were tested to determine whether a blend of 70:30, helium:oxygen (heliox) improved FEV1. No pretreatment with bronchodilators occurred. The change in absolute and predicted FEV1 was measured after five minutes of breathing heliox. It was found that the absolute FEV1 during heliox was not significantly different from the absolute FEV1 before heliox. The difference between the absolute FEV1 during and before heliox was found to be -0.04 l. The percent predicted FEV1 during heliox was 40.8% +/- 13.0% vs. 41.5% +/- 11.9% before heliox. The difference between the percent predicted FEV1 during and before heliox was found to be -0.7%. We conclude that 70:30 heliox does not improve FEV1 in these patients.