Asthma is a serious health problem, with both mortality and severity increasing in developed countries (55-57). The development of more specific tests to measure the prevalence and incidence of asthma is of paramount importance. Because measurement of BHR is the only practical objective method for assessing the abnormality of the airways in asthma, it is important to continue examining this measurement as a means of detecting asthma and to explore ways of increasing the sensitivity and specificity of the test. At present, it is clear that measurements of BHR in the community cannot be used as a marker of asthma and that the test requires careful interpretation, especially in adult populations. However, it is becoming apparent that moderate and severe levels of BHR (PD20FEV1 less than 0.8 mumol of histamine) are almost always associated with clinically important morbidity, and comparison of this BHR severity in populations may yield more information than comparison of a wider range of severity in which the associated factors are not well-defined. It is important to determine whether this range of severity of BHR is associated with long-term morbidity from asthma, and whether the use of other measurements from the dose-response curve, such as the shape or similarity of curves to different provoking agents, can increase the specificity of the test. It is also important to continue to investigate the risk factors associated with the development and occurrence of BHR so that individuals who are likely to develop BHR can be identified and, in the longer term, preventive measures can be introduced. There have been relatively few studies of the prevalence of BHR in large populations of children and adults in North America. Such studies would provide essential comparative information about apparent regional differences in both the prevalence of and the mortality from asthma.