Clinical features and the attack mechanism of Aspirin-induced asthma (AIA) were studied in 31 AIA cases (9.3%) among 333 asthmatics. AIA was defined positve from the anamnesis or from a positive aspirin inhalation challenge test result. AIA had more severe asthma symptoms than non-AIA. Moreover. the frequency of nasal polyp in AIA patients was higher than that in non-AIA ones. To clarify the mechanism of AIA, the concentrations of plasma histamine and leukotrienes were measured before and after the aspirin inhalation challenge test. There was no difference in the plasma histamine leved between AIA and non-AIA patients. The concentrations of plasma histamine and leukotrienes were meaurred before and after addition of aspirin to the peripheral blood in vitro. There were no differnces in the plasma histamine or leukotriene levels between AIA and non-AIA patients. These findings suggest that the mast cell is less related to the mechanism of AIA. Therefore, it may be necessary to examine mediators other than histamine and leukotrienes.