Asthma has been defined as variable airflow obstruction. The symptoms of asthma are not specific for the condition and, therefore, the presence of variable airflow obstruction needs to be objectively confirmed. When airflow obstruction is present and when it can be completely reversed with treatment, confirmation of the diagnosis is easy. When spirometry is normal or when there is chronic airflow limitation, however, other methods of investigation are required. These methods include inhalation tests with histamine or methacholine, diurnal variation of peak flow rates and tests with exercise or hyperventilation. When spirometry is normal, methacholine or histamine tests appear to be the most sensitive method but, if results are normal, they do not exclude past or future asthma. However, when there is chronic airflow limitation neither variable airflow obstruction nor methacholine or histamine hyperresponsiveness are specific for asthma; tests with hyperventilation (and probably others that act through mediator release) may be more specific. The presence of asthma probably requires the release of chemical mediators, from mast cells and other cells, which can produce inflammation. This has been particularly studied after inhalation of allergens or chemical sensitizers which can trigger both early and late asthmatic responses. The late responses are associated with prolonged increases in airway responsiveness and are considered to be a result of the cellular phase of inflammation. It is possible that persistent or recurrent inflammation, due to these and other stimuli, is the cause of the persisting airway hyperresponsiveness and variable airflow obstruction in asthma, as well as the other features of cough and sputum, but this requires further investigation.(ABSTRACT TRUNCATED AT 250 WORDS)