Distribution of bronchial hyperresponsiveness to methacholine was assessed in 791 consecutive patients who were referred to the outpatient clinic of the pulmonary department due to asthmatic or persistent lower airway symptoms. Bronchial asthma was diagnosed in 319 patients. Clinical sensitivity of methacholine challenge for the disease was 89 percent and specificity, 76 percent. The degree of bronchial hyperresponsiveness in the entire group of asthmatic patients was unimodally log normal distributed. Of the 82 patients with allergic rhinitis without concurrent asthma, 27 percent had bronchial hyperresponsiveness, but of a markedly lesser degree than in the hyperresponsive asthmatic patients. In 49 patients with chronic bronchitis, 22 percent had hyperresponsiveness. The present data indicate that the degree of bronchial hyperresponsiveness in asthmatic patients is unimodally distributed, supporting the view that both genetic and environmental factors have an impact upon its development. Although the degree of bronchial hyperresponsiveness in asthma is more pronounced than in allergic rhinitis or in chronic bronchitis, a marked overlap exists.