The role of inhaled corticosteroids (ICS) on disease progression in asthmatic children is not yet clear. The aims of the thesis were to assess if ICS treatment had an effect on lung function in young asthmatic children and to explore if early ICS treatment during the first 2 years of age can modify occurrence of later asthma. Furthermore, a severity score based on severity and frequency of obstructive airways disease (OAD) by 2 years of age was assessed to predict later asthma.Lastly, the purpose was to assess the prevalence of asthma in school children. The present study is part of a 10 year follow-up of children in the prospective birth cohort, the Environment and Childhood Asthma study in Oslo, started in 1992. The children who participated in a nested case-control study during the first 2 years were primarily studied.Lung function improved in children using ICS treatment until 2 years of age. However, use of ICS during the first 2 years of life in children with OAD did not reduce later asthma. Severity score of OAD by 2 years of age strongly predicted asthma at 10 years of age. We also found that 20.2 % 10-yr old children in Oslo at some time had asthma.