Bronchial hyperresponsiveness (BHR) produces the characteristic pathological abnormalities seen in asthma and clearly plays a central role in the pathophysiology of asthma. The presence of BHR has been demonstrated in infants with asthma, as has the possibility of BHR persisting through the childhood period. The level of BHR may not only reflect the state of the airways, as a marker of airway dysfunction, but may also predict the persistent prognosis of the disease. Thus, measurement of BHR may provide important information about the symptoms and lung function in children with asthma. In view of multiple pathophysiological mechanisms, BHR does not seem to have a single cause. Many potential confounding variables, such as age, gender and genetic status, and some environmental factors, such as allergens, infections, and pollutants, could be responsible for the establishment of childhood BHR. There may be differences between the mechanisms that induce transient BHR and the mechanisms that induce persistent BHR. Also, there may be differences between the causes that induce BHR in the infantile period and the causes that maintain persistent BHR during childhood asthma. There is also disagreement as to the most suitable method to measure BHR in children, especially in infants. The assessment of BHR in young children has not been uniformly successful, and measurements of BHR changes over the childhood period (are associated with a number of problems. To resolve these problems, there may be two ways to study childhood BHR. One is to use age-matched specific techniques to clarify the precise BHR in each age group; the other is to use simple techniques that can be performed over the childhood period on a large number of subjects. In studies of infantile respirator, dysfunction the ultimate goal is to establish a simple, noninvasive method by which measurements of respiratory function may be obtained in infants. Further investigations and acceptable methods will be needed to clarify, the mechanisms involved in the establishment of asthma throughout the childhood period.