Asthma is characterized by the development of reversible airway obstruction and the decline of pulmonary function test. Determination of baseline ventilatory parameters i.e. forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) with airway obstruction reversibility test using spirometry play essential role in the diagnosing and monitoring asthma. Peak expiratory flow (PEF) in monitoring of asthma course is also recommended, but this parameter is less sensitive than FEV1. The fundamental feature of asthma is bronchial hyperreactivity. The term bronchial hyperreactivity indicates a degree of responsiveness greater than that observed in nonasthmatic subjects with normal spirometry. Bronchial hyperreactivity can be defined using inhaling provocative test with bronchospastic substances (histamine, methacholine, 5-AMP). In children very useful bronchospastic stimulus is exercise which is characterized by very great specificity but very low sensitivity. Treatment of asthma results not only in clinical course improvement, but also in increase in ventilatory parameters which allow us to determine clinical stadium of asthma. In asthmatic patients with bronchial remodeling an increase in ventilatory parameters is not observed. Bronchial remodeling results irreversible airway obstruction and is typical in patients suffering from severe asthma. In this kind of asthma is characterized by negative result of airway obstruction reversibility test and sometimes by low value of PEF. Bronchial hyperreactivity is very useful in diagnosing process. Its value is connected with bronchial allergic inflammation and with pulmonary function tests. Treatment of asthma although improves clinical course of the disease and ventilatory parameter has no influence on bronchial hyperreactivity.