During the second half of the 20th century, the increasing prevalence, morbidity, economic burden, and, in some countries, mortality from asthma have generated worldwide concern. The prevalence in the United States and other English-speaking countries is higher than that in most other countries, but worldwide variations cannot be explained by current knowledge of recognized risk or protective factors. According to hospital admission rates, asthma morbidity rates have also risen throughout the world during the past 40 years. These trends are likely due to many different factors, including an increase in the prevalence of severe asthma. Asthma mortality rates gradually declined in the United States during the 1960s and 1970s but have exhibited a substantial, progressive increase during the past 20 years. This trend stands in contrast to those in most other western countries, where asthma mortality rates have generally been decreasing during the 1990s. In both western and developing countries, the considerable economic burden of asthma disproportionately affects individuals with severe disease. This observation illustrates the potential for reducing the costs associated with asthma through management approaches that have been proven to reduce morbidity and mortality.