Atopic dermatitis often occurs in patients who have high IgE levels and positive immediate skin tests to several common allergens. However, there is considerable doubt about the role played by allergens in this disease. Patch testing for 48 h at superficially abraded skin sites revealed that allergens could induce eczematous lesions in atopic dermatitis patients but only in those who also gave a positive immediate skin reaction to the same allergen. Lesions induced by the purified house dust mite antigen, antigen P1 contained mononuclear cells, basophils, eosinophils, and neutrophils. These patients also had raised specific serum IgE against antigen P1, and their leucocytes released histamine upon exposure to the same antigen. Thus an acute eczematous lesion can be induced by the application of inhalant allergens to the skin.