This study examines the distribution of intraepithelial dendritic cells in eight atopic patients with symptomatic asthma and their ability to induce activation of autologous T lymphocytes in vitro. All subjects were sensitized to Dermatophagoides pteronyssinus. The incubation of asthmatic epithelial cells and dendritic cells with autologous resting CD4-positive T cells and purified extracts of D pteronyssinus induced T cell activation and release of high levels of interleukin-4 (IL-4) and interleukin-5 (IL-5). The antigen-presenting activity of dendritic cells was potentiated by epithelial cell-derived granulocyte-macrophage colony-stimulating factor (GM-CSF), since an antibody against GM-CSF reduced it. Circulating monocytes of the two groups of donors were equally effective in promoting selective activation of IL-4- and IL-5-producing T cells. Thus, an interaction between dendritic cells and allergens may favor local activation of CD4-positive T cells with Th2-like function in atopic asthmatic subjects, thereby promoting the expression of the disease.