Asthma is an increasingly serious cause of morbidity and mortality in the United States, affecting approximately 12 million people, including men and women, children and adults, and all racial and ethnic groups. It is now recognized that asthma is a complex disease of varied etiology triggered by a number of factors such as allergens, drugs, chemicals, exercise, cold dry air, infections, and emotions. Asthma is a chronic disease requiring multiple medications to treat and control symptoms as well as medications thought to control the underlying inflammation. Despite major advances in understanding the etiology and pathophysiology of asthma and the development of new therapeutic modalities, the prevalence, severity, and mortality from asthma have all increased over the past decades in all age groups. Hospitalizations for asthma have doubled in adults and increased fivefold for children over the past 20 years. Mortality appears to be particularly high in urban and rural minority populations. Asthma continues to place a heavy burden on patients and their families as well as the health-care system. In an attempt to respond to the need for well-designed clinical trials to allow rapid evaluation of new and existing therapeutic approaches for asthma and for dissemination of laboratory and clinical findings to the health-care community, the Division of Lung Diseases, National Heart, Lung, and Blood Institute, established the Asthma Clinical Research Network.