We determined whether chronic administration of furosemide aerosol would be beneficial for the treatment of asthma. First, we showed that furosemide aerosol delivered from a metered-dose inhaler (10 and 20 mg) significantly protected against sodium metabisulfite (MBS) challenge by 0.6 and 1.3 doubling dilutions respectively in 12 volunteers with mild asthma. In a double-blind cross-over study, we examined the effect of furosemide aerosol from a twice more efficient metered-dose inhaler (10 mg four times per day) inhaled over 4 wk versus placebo in 12 other asthmatic subjects. There was no significant effect of furosemide on bronchial responsiveness to methacholine or MBS. Treatment with furosemide over 1 mo did not improve bronchial hyperresponsiveness in subjects with mild asthma.