Vascular reactions to mechanical stroking, topical application of nicotinic acid ester, and methacholine chloride were examined in both the normal and abnormal skin of 100 patients with atopic dermatitis and 20 patients with allergic contact dermatitis. White dermographism, nicotinic acid blanching, and delayed blanch with methacholine consistently occurred in areas of skin with eczematous change of patients with atopic dermatitis and those with allergic contact dermatitis. Normal skin of atopic patients did not show the abnormal vascular reactions. It is suggested that white dermographism, nicotinic acid blanching, and delayed blanch with methacholine seen in atopic dermatitis are secondary phenomena that give no definite information concerning the diagnosis of this disease.