The protective effect of different doses of sodium cromoglycate (4 mg, 12 mg and 20 mg) given as pressurized aerosol on fog-induced bronchoconstriction were investigated in three different groups of asthmatic patients. Premedication with the active drug was compared versus absence of premedication. The authors discuss their results by a statistical analysis of the FEV1 variations. Sodium cromoglycate effectively prevents fog-induced bronchoconstriction with a clear dose-dependent effect.