There is now clear evidence that asthma prevalence increased significantly, especially in developed countries, during the second part of the 20th century. What caused this increase is currently unknown. Recent reports from the United States and the United Kingdom suggest that asthma prevalence may have plateaued between 1995 and the first few years of the present century. This stabilization, and even some decrease in asthma prevalence, especially in countries with high baseline rates, was confirmed by the International Study of Asthma and Allergies in Children. The hospitalization rate for asthma (as a proportion of asthma patients) decreased significantly in the United States between 1980 and 1995, then remained stable between 2001 and 2004. However, the asthma death rate (as a proportion of subjects with asthma) did not decrease significantly during either of those periods. A better understanding of what determines the stable asthma death rate is urgently needed, especially since inhaled corticosteroids have been shown to prevent asthma deaths in persons who take them regularly.