Influence of cigarette smoking on the pathogenesis of asthma in the elderly remains controversial. This study was undertaken to clarify the influence of cigarette smoking on IgE-mediated allergy, bronchial hyperresponsiveness, and the generation of leukotrienes B4 (LTB4) and C4 (LTC4) by leukocytes in 48 elderly patients (25 patients with a history of smoking for more than 20 years and 23 never-smokers) with asthma over the age of 70 years. 1. The incidence of positive IgE antibodies for inhalant allergens was significantly higher in asthmatics with a history of smoking than in never-smokers of asthmatics. 2. In bronchial hyperresponsiveness for methacholine, the Cmin was significantly lower in ever-smokers than in never-smokers of elderly asthmatics. 3. The generation of LB4 was significantly increased in patients with a history of smoking compared with those without a smoking history. However, the difference in LTC4 generation was not significant between ever-smokers and never-smokers of asthmatics. These results suggest that cigarette smoking increases bronchial hyperresponsiceness through IgE-mediated allergy and LTB4 generation.