The morbidity and mortality due to asthma in the Western world have increased 75% in the past 20 years. Recent studies have demonstrated that sensitization to cockroach allergens correlates strongly with the increased asthma morbidity for adults and children. We investigated whether dexamethasone administered before or after allergen challenge would inhibit the pulmonary inflammation and airway hyperresponsiveness in a mouse model of asthma induced by a house dust extract with high levels of cockroach allergens. For the prevention experiment, mice were treated with an intraperitoneal injection of dexamethasone 1-hour before each pulmonary challenge and airway hyperresponsiveness was measured 24-hour after the last challenge. Mice were sacrificed 48-hour after the last challenge. For the reversal study, airway hyperresponsiveness was measured 24-hour after the last challenge and the mice were treated with dexamethasone. Dexamethasone treatment before allergen challenge significantly reduced the pulmonary recruitment of inflammatory cells, myeloperoxidase activity in the lung, airway hyper-reactivity, and total serum IgE levels as compared with PBS-treated mice. Additionally, dexamethasone treatment could significantly reduce the airway hyperreactivity of an established asthmatic response. These results demonstrate that dexamethasone not only prevents, but also halts the asthmatic response induced by house dust containing cockroach allergens. This model exhibits several features of human asthma that may be exploited to the study of pathophysiologic mechanisms and potential therapeutic interventions.