We previously reported in ovalbumin-induced model of allergic asthma that Flt3-L reversed airway hyperresponsiveness (AHR) and airway inflammation, and increased the number of regulatory CD11c-^highCD8-^highCD11b-^low dendritic cells in the lung. In this study, we investigated the effect of Flt3-L in a clinically relevant aeroallergen-induced asthma on the phenotypic expression of lung T-cells. Balb/c mice were sensitized and challenged with cockroach (CRA) and AHR to methacholine was established. These mice received three i.p. injections of anti CD25 antibody (PC61) (250 µg) and Flt3-L (3µg) daily for ten days. Cytokines and immunoglobulins levels in the serum were measured and differential BALF cell counts were examined. Flt3-L reversed AHR to methacholine to the control level. Flt3-L significantly decreased levels of BALF IL-5, IFN-, eosinophilia and substantially increased IL-10 and the number of CD4⁺CD25⁺Foxp3⁺IL-10⁺ T-cells in the lung. Administration of PC61 antibody blocked the effect of Flt3-L and substantially increased AHR, eosinophilia, BALF IL-5 and IFN- levels and decreased BALF IL-10 levels and the number of CD4⁺CD25⁺Foxp3⁺IL-10⁺ T-cells. Flt3-L significantly decreased CD62-L but, increased ICOS expression and Foxp3 mRNA expression in the CD4⁺CD25⁺ T-cells isolated from lungs of Flt3-L treated CRA-sensitized mice compared to without Flt3-L treatment and PBS control group. Flt3-L significantly inhibited the effect of CRA sensitization and challenge to increase GATA3 expression in lung CD4⁺CD25⁺ T-cells. Collectively, these data suggest that the therapeutic effect of Flt3-L is mediated by increased density of naturally occurring CD4⁺CD25⁺Foxp3⁺IL-10⁺ICOS⁺ T-regulatory cells in the lung. Flt3-L could be a therapeutic strategy for the management and prevention of allergic asthma.