Seventy-eight children who had a history of asthma were studied while they were symptom-free. There was a highly significant correlation between the dose of aerosolized histamine that produced a decrease in FEV1 of 20% and each of the features in the history that indicated severity of asthma. The correlation was strengthened by the combination of these features into a weighted asthma history score. None of the subjects with mildly increased bronchial reactivity had a history score of severe asthma, and none with markedly increased bronchial reactivity had mild asthma. There was also a highly significant correlation between histamine dose and the results of spirometric tests for airway obstruction. However, the correlation between asthma history score and provocative histamine dose was highly significant even in the 21 subjects who were apparently free of airway obstruction at the time of testing. Furthermore, the correlation between asthma history score and histamine dose was stronger than that between asthma score and any spirometric test, indicating that the histamine test more accurately assessed the overall severity of the asthma. Measurement of bronchial responsiveness to histamine is a useful adjunct to history in determining the severity of asthma in an individual and should be considered as an objective way of grading subjects according to severity of asthma in a clinical study.