Skin eosinophilia induced by grass pollen or platelet-activating factor (PAF) was investigated in patients with allergic and nonallergic asthma and atopic dermatitis (AD) and in normal control subjects. Intracutaneous injection of PAF resulted in positive immediate skin reactions in all tested individuals. Since most of the patients with allergic and nonallergic asthma and eczema under investigation were characterized by a peripheral blood eosinophilia, we investigated whether PAF should be capable of mobilizing blood eosinophils to the skin of patients with nonallergic asthma and AD using the Rebuck skin window technique. Our results demonstrated that, in allergic individuals, allergen induced a much stronger skin eosinophilia than PAF and in a larger percentage of the patients (77% versus 45%). There was no correlation between skin eosinophilia induced by PAF and the occurrence of late skin reactions to allergen. Furthermore, the PAF-induced and the allergen-induced skin eosinophilia differed histologically. In nonallergic individuals, neither allergen nor PAF was capable of mobilizing eosinophils in the skin window, in spite of the presence of peripheral blood eosinophilia. Our results indicate that the allergen- and PAF-induced skin eosinophilia are different and that in patients with nonallergic asthma and AD, another chemoattractant than PAF must be responsible for eosinophil mobilization in the target organ.