An easy and safe dose-response histamine-inhalation test is described, to measure the level of non-specific bronchial reactivity. The test was performed in 307 subject. Non-specific bronchial reactivity was increased in 3% of presumed normal subjects, in 100% of active asthmatics and in 69% of asymptomatic asthmatics with previous symptoms only at times of exposure to clinically relevant allergens. It was also increased in 47% of patients with cough and no other chest symptoms, in 40% of patients with rhinitis and vague chest symptoms not by themselves diagnostic of asthma, and in 22% of patients with rhinitis and no chest symptoms. The patients with asthma were studied when their asthma was well controlled and when their minimum drug requirements had been established. The mean level of bronchial reactivity increased with increasing minimum drug requirements. The level of bronchial reactivity also showed a strong negative correlation with the forced expiratory volume in 1 sec (FEV1). Atopic subjects, with or without asthma, showed a significant positive correlation between the level of bronchial reactivity and atopic status as indicated by the number of positive allergy skin tests.