According to the Centers for Disease Control and Prevention, the prevalence of asthma among U.S. children increased from 3.6% in 1980 to 5.8% in 2003. Asthma is the third leading cause of hospitalization among persons under 18 years of age in the United States, exceeded only by pneumonia and injuries.1 Increases in the prevalence of asthma of similar or even greater magnitude were reported from other countries during the second half of the 20th century (Fig. 1A). For example, in 1964, 19% of Australian children were reported by their parents to have had asthma or wheezing at some time during their first 7 years of life; in 1990, such symptoms were reported for 46% of children.52 For many countries, there are no data on temporal changes in the prevalence of asthma before the 1990s (Fig. 1A). After the 1990s, estimates of temporal trends in the prevalence of asthma in several European and Asian countries are conflicting. In some populations, the prevalence of diagnosed asthma is still rising, whereas in others it appears to be stable or decreasing slight- ly (Fig. 1A). There are no clear differences in trends in prevalence between children and adults, between severe and mild asthma, or between developed and developing countries; however, there are few studies from developing countries.