Extensive surveys have shown that 6-20 % of children have acute expiratory wheezing (1, 2). The lifetime prevalence of wheezing among 12–14-year-old children in the Western countries is 8-19 % (3, 4). The clinical illness is called bronchiolitis when occuring for the first time in infants younger than 12 months (5). When occuring later or recurring, beside bronchiolitis a diagnosis of wheezy bronchitis has been used (6). In older children wheezy bronchitis cannot be distinguished from virus-induced-asthma. In many countries the first expiratory wheezing attack at the age of > 3 years is diagnosed as asthma. To some extent, bronchiolitis, wheezy bronchitis, and asthma are expressions of the same pathologic process and at present there are no rigid criteria to separate these three illnessess.