Asthma is a chronic inflammatory disease of the airways involving a characteristic pattern of airway infiltration with lymphocytes, eosinophils, and mast cells, subepithelial deposition of collagen, and hypertrophy and hyperplasia of smooth muscle and of goblet cells and submucosal glands. The consequences of this chronic process include episodic or persistent symptoms, bronchial hyperreactivity, attacks of bronchoconstriction that may require emergency care or hospitalization and can lead to death, impairment in quality of life, and the development of irreversible airflow obstruction. Careful pathologic studies have shown that inhaled corticosteroid therapy can reverse or suppress airway inflammation, and prospective controlled clinical trials have proven that it can also improve symptoms, reduce bronchial hyperreactivity, and reduce the frequency and severity of attacks. It is also highly likely, although it is not yet proven, that inhaled corticosteroid therapy reduces the risk of asthma fatality and prevents or retards airway wall remodeling. These beneficial effects are easily shown in patients with moderate or severe asthma. Although inhaled corticosteroid therapy also benefits patients with mild asthma, it is less certain that the costs and risks of continuous therapy are justified. For these patients, the most important issues that remain to be resolved are the nature of the risk of development of permanent airflow obstruction and the effects of early, sustained treatment on the chances of sustained remission of asthma after all therapy has been stopped.