Background: It is accepted that T regulatory cells (Treg) control different types of immune responses. In connection with this role, we have recently described an important increase in CD4+, CD25(high), Foxp3+ lymphocytes in the airway system of horses coursing with an exacerbation of severe equine asthma (EA). To explore the potential role of this population in the resolution of EA inflammation, we used a murine experimental model in which airway neutrophilic inflammation, which is similar to that observed in EA, is induced in mice by continual exposure to Aspergillus fumigatus contaminated hay. This model has the advantage that in mice we may induce a reduction of the Treg population using low doses of cyclophosphamide (Cy). Results: The results indicated that the percentage of Treg cells increased with allergen exposure, as in horses; and animals partially depleted of Treg cells by treatment with Cy showed increased airway inflammation, demonstrated by an increased percentage of neutrophils and specific immunoglobulins in bronchoalveolar lavage fluid (BALF). Furthermore, a histopathologic study of animals that were pretreated with Cy before antigenic challenge showed higher cellular infiltration in the lung and deeper remodeling changes in the bronchi, including epithelial and goblet cell hyperplasia as well as airway smooth muscle hypertrophy. Conclusion: In this murine model of EA, the reduced number and function of Treg induced by low doses of Cy, which directly correlates with increased airway inflammation and lung infiltration, indicates that Treg may play a major role in the regulation and resolution of EA.