The effect of pretreatment with inhalation powder containing fenoterol, sodium cromoglycate, fenoterol plus sodium cromoglycate and placebo, in the prevention of exercise-induced asthma was compared in a randomized, double-blind, cross-over investigation including 17 asthmatic children with exercise-induced asthma. Six minutes of treadmill running was carried out in a climate chamber 30 minutes after pretreatment on the four test days. The post-exercise fall in PEF after pretreatment with fenoterol was 7%, with sodium cromoglycate 20%, with fenoterol plus sodium cromoglycate 6% and after placebo 27%. Both sodium cromoglycate and fenoterol provided protection against exercise-induced asthma (p less than 0.05 and p less than 0.001). Fenoterol gave significantly better protection than sodium cromoglycate (p less than 0.001), and it is concluded that the treatment of choice for the prevention of exercise-induced asthma is the inhalation of a beta 2-agonist.