Significant advances have been made in recent years in understanding the different ways in which asthma is expressed during childhood. There is a large group of children (20% of the population) who present with wheezing during viral infections during the first 3 years of life but whose symptoms disappear after this age. These children do not have any of the important risk factors associated with asthma in later childhood (high serum IgE levels, bronchial hyperresponsiveness, family history of asthma). The main characteristic associated with this condition is reduced lung function, which can be detected before these illnesses develop. It has been suggested that there is nothing unusual or characteristic in the immune response to viruses in these children, but it would appear that these children have narrower intrapulmonary airways. A second group of young children (10% of the population), who also wheeze during viral infections, present with most of the risk factors associated with asthma in later childhood. In this group of children symptoms tend to persist with age. It appears that in these children the immune response to viruses is characterized by a predominance of T H 2-like cells. This suggests that their early wheezing is the expression of asthma because a T H 2-like response to allergens and viruses is a characteristic found in most asthmatic children. A better understanding of the different types of childhood asthma, together with development of accurate prognostic tests, may be decisive in determining adequate strategies for prevention and treatment of the condition.