T-cell lines have been derived from aeroallergen-induced eczematous patch test sites of patients with atopic dermatitis (AD). Biopsy specimens were obtained 24 hours after allergen application to the skin, and only in vivo-activated T cells were propagated. From one patient, the T-cell lines were subcloned at 0.3 cells per well. With allergen-induced interleukin (IL) production, all clones tested (n = 13) were found to be specific for the allergen, producing IL-4 and granulocyte-macrophage-colony-stimulating factor. No IL-2 or interferon-gamma was found. The allergen-specific proliferative response of these clones, although the response was dependent on exogenous IL-2 or IL-4, also proved that all clones were allergen specific. Under optimal stimulation (aCD3 plus phorbol myristate acetate), 15% of the clones appeared to be of Th0 phenotype and 70% of Th2 phenotype. In 15% of the clones, IL-4 was produced in the absence of IL-2, IL-5, or interferon-gamma. Supernatants of all clones tested induced IgE production by B cells from normal non-atopic donors. The T-cell lines of the other patient demonstrated similar results; allergen-specific proliferation was dependent on exogenous IL-2 or IL-4 and stimulation with aCD3 plus phorbol myristate acetate demonstrated that the T cells in these lines were of the Th2 phenotype. In conclusion, our data reveal that, in AD, percutaneous sensitization to aeroallergens may occur and indicate that allergen-specific Th2 type T cells may be responsible for the high levels of (specific) IgE found in 80% of patients with AD.