Repeated bronchial histamine challenges before, during, and after the birch pollen season were performed in 22 allergic patients with bronchial hyperreactivity (BHR) treated for 6 wk with sodium cromoglycate (SCG), 20 mg, four times a day, or placebo in a double-blind, randomized group comparison. Clinical assessments of the asthmatic symptom score and peak expiratory flow revealed less symptoms and less use of bronchodilators in the SCG group. Responsiveness to histamine was significantly increased in the placebo group after 14 days with high pollen counts. After the season there was an immediate return to preseasonal value. There was no change in responsiveness in the SCG group, demonstrating significant protection against pollen-induced increase of BHR. The results support the hypothesis that inhibition of mediator release, which is demonstrated for SCG, leads to a reduction of the nonspecific BHR.