The protective effect of sodium cromoglycate against the bronchospasm induced by exercise in 10 asthmatic subjects was compared with that of placebo in a single-blind study. The drug, which was administered by means of a metered dose aerosol, at a lower dose than is currently used in the therapy of asthma, was shown to be significantly more effective than placebo in preventing postexercise changes in all the respiratory function parameters measured (VC, FEV1 PEFR, MEFR50, TGV and SGaw)