Periodic exacerbations of disease severity, which may lead to hospitalization, are a characteristic feature of asthma and chronic obstructive pulmonary disease (COPD), becoming more prevalent as disease severity increases. Oral corticosteroids increase the rate of resolution of these episodes in both diseases. Inhaled corticosteroids are much less effective at conventional doses and are not recommended as a primary treatment for exacerbations of either disease. Maintenance therapy with inhaled corticosteroids significantly reduces the chance that a further exacerbation will occur in asthma. In general, increasing doses of inhaled corticosteroids are more effective than placebo therapy in preventing exacerbations, at least until patients become persistently symptomatic and regular users of inhaled corticosteroid therapy. Thereafter, the gains from doubling the dose of inhaled corticosteroid maintenance therapy are modest and generally inferior to those that result from adding other antiinflammatory or bronchodilator agents to the treatment regime. The reduction in the incidence of exacerbations with inhaled corticosteroids, compared with placebo, ranges from 15 to 20% in COPD versus almost 50% in severe asthma. However, given the impact of exacerbations on overall quality of life in COPD, even this modest reduction is likely to be clinically important.