Asthma has emerged as a major public health problem in the United States over the past 20 years. Currently, nearly 15 million Americans have asthma, including almost 5 million children. The number of asthma cases has more than doubled since 1980. Approximately 5,500 persons die from asthma each year, and rates have increased over the past 20 years. Rates of death, hospitalization, and emergency department visits are 2-3 times higher among African Americans than among white Americans. The costs of asthma have also increased to 12.7 billion dollars in 1998. Both lifestyle and environmental hypotheses have been invoked to explain the increase in asthma prevalence. Several studies have examined the relationship of obesity and asthma and found associations suggesting that obesity predisposes to the development of asthma. Some studies have found that day care attendance and having older siblings protect against the development of asthma. This observation has led investigators to hypothesize that increased exposure to microbial agents might protect against asthma (the hygiene hypothesis). Environmental exposures found to predispose to asthma include house dust mite allergen and environmental tobacco smoke. Although current knowledge does not permit definitive conclusions about the causes of asthma onset, better adherence to current recommendations for medical therapy and environmental management of asthma would reduce the burden of this disease.