It is difficult to analyse the relationship between bronchial responsiveness and the severity of asthma since each alone is difficult to assess. Asthma is a heterogeneous condition with many patterns of expression, and clinical methods used to assess its severity have their limitations. Problems also arise in interpreting the results of bronchial provocation tests and methodological differences make comparisons between studies difficult. The findings of both cross-sectional and longitudinal studies have shown a general relationship between the degree of responsiveness and the severity of asthma, but within subjects the relationship is weaker. A greater understanding must await increased knowledge of the mechanisms underlying asthma and the contribution which hyperresponsiveness makes to each of these.