Inhaled corticosteroids are now recommended as first-line therapy for asthma. Although these drugs clearly improve the symptoms of the disease and the associated physiologic abnormalities, epidemiologic studies provide important information on their effectiveness in preventing asthma morbidity and mortality. We review the evidence regarding the role of inhaled corticosteroids in the prevention of asthma fatality and hospitalization. In the process, we discuss the methodologic complexities of the nonexperimental studies and the implications of the methodologic issues on the evaluation of the impact of these drugs. Eight of the cohort and ecologic studies conducted to date strongly suggest that inhaled corticosteroids, when taken regularly, decrease the number of hospitalizations for asthma by up to 80%. For asthma death, the results of 11 investigations appear less consistent, especially those of several cohort and case-control studies whose principal objective was to examine not the benefit of inhaled corticosteroids but the adverse effects of other drug classes. Much of the inconsistency in the results, however, can be explained by weaknesses in study design and analysis-in particular, the failure to consider exposure in terms of regular use of inhaled corticosteroids. When the most recent study involving the use of the Saskatchewan databases is considered, it is evident that regular treatment with conventional or low-dose inhaled cortico-steroids results in a significant reduction in fatalities due to asthma. In all, the evidence to date strongly indicates that regular use of inhaled corticosteroids, even at low doses, would prevent the major portion of asthma hospitalizations and deaths.