An increased understanding of the causes of asthma is coming from the international comparisons of asthma prevalence, particularly those from the European Community Respiratory Health Survey of asthma prevalence in adults and the International Study of Asthma and Allergies in Childhood. From these and other studies of asthma prevalence, it is possible to draw some tentative conclusions as to the patterns of asthma prevalence worldwide. There are five striking patterns: first, asthma prevalence is increasing worldwide; second, asthma is generally more common in Western countries and less common in developing countries; third, asthma is more prevalent in English-speaking countries; fourth, asthma prevalence is increasing in developing countries as they become more Westernized or communities become urbanized; and fifth, the prevalence of other allergic disorders may also be increasing worldwide. These five key features of the international patterns of asthma prevalence raise major questions about the role of "established" risk factors for the development of asthma. As a result, recent research has expanded to include the study of novel factors that may "program" the initial susceptibility to sensitization or contribute to the development of asthma independent of atopic sensitization. These include various exposures in utero, which are reflected in various perinatal factors measured at birth, and exposures (or lack of exposures) in the early years of life that may make the infant more susceptible to the subsequent development of asthma. These issues are now the focus of an intensive research effort worldwide, and the next few years are likely to see exciting advances in our understanding of the causes of asthma.