The prevalence of asthma and allergic diseases has been increasing particularly in children over the last decades in Western countries. The German reunification allowed to study the effect of 40 years of different environmental and social development on asthma and allergy prevalence in two genetically homogenous populations. In children and young adults, the prevalence of asthma, hay fever and skin test reactivity to common aeroallergens was considerably higher in West Germany as compared to East Germany. The increased prevalence in asthma was explained by the higher allergic sensitization in West Germany. Exposure to SO2, particulate matter and cars was associated with bronchitis, nonspecific respiratory symptoms and decrements in pulmonary function, but not with asthma and allergy. Atopy was found to be the major factor associated with the inception of asthma. Children who had lived during their first three years of life in East German living conditions and had thereafter been exposed to a rapid Westernization of their environment and lifestyle, showed increased atopy and hay fever while bronchitis decreased, but asthma and bronchial hyperresponsiveness remained unchanged. This points toward a period during early life which determines the development of asthma and which seems to differ from the factors responsible for allergic sensitization. Since East European living conditions were similar to those in the West decades ago, the same factors could be responsible for the increased allergic sensitization in West Germany and the secular increase in atopic diseases and asthma in the West. Factors associated with the more modern life style may account for this difference, while the increased levels of SO2 and dust exposure in East Germany were obviously no risk for the development of childhood asthma or allergic sensitization.