The prophylaxis of exercise-induced asthma with inhaled formoterol (12 micrograms) was compared with inhaled salbutamol (200 micrograms) and placebo in 12 patients with atopic asthma. Both drugs produced equal bronchodilation 2 and 4 h after administration. Both drugs protected equally against exercise-induced bronchoconstriction 2 h after administration; at 4 h, formoterol gave undiminished protection from that seen at 2 h while salbutamol was no more effective than placebo.