The effect of cromolyn sodium (CS) pressurized aerosol on bronchial hyperreactivity was assessed by comparison with placebo in a double-blind crossover study of 14 adult patients with clinically stable asthma. The trial was performed in a cold climate during the pollen-free winter months and the patient's risk of exposure to clinically relevant allergens was judged to be low. The dose was two puffs, each of 1 mg CS or placebo, four times daily over two successive 4-week periods, the order of treatment being decided by random allocation. No significant difference between treatments was observed in bronchial reactivity to histamine, determined as PC15. There was no difference between treatments with regard to symptoms, which were slight, or daily peak expiratory flow recordings, which showed minimal circadian variation. The results suggest that, although prolonged treatment with CS may decrease bronchial reactivity by reducing airway inflammation secondary to the assault of allergic stimuli, the drug probably has little or no effect on the basal bronchial reactivity in asthma.