The influence of cigarette smoking on the pathophysiology of asthma in the elderly remains controversial. In this study, the inluence of cigarette smoking on IgE - mediated allergy including the generation of leukotrienes B4 (LTB4) and C4 (LTC4) was examined in forty asthmatics over the age of 70 years (20 ex-smokers and 20 never-smokers), and 20 patients with pulmonary emphysema over age 70 (all ex - smokers). The frequency of patients with serum IgE more than 200 IU/ml was significantly larger in smoking asthmatics than in non - smoking asthmatics. The incidence of patients with positive RAST score for inhalant allergens, was also significantly higher in patients with a history of smoking than in those without a smoking history. There were no significant differences in the frequency of patients with positive RAST and those with serum IgE more than 200 IU/ml between non-smoking asthmatics and patients with pulmonary emphysema. The generation of leukotriene B4 (LTB4) by leukocytes was significantly more increased in ex-smokers than in never-smokers in the elderly asthmatics. The results suggest the possibility that cigarette smoking enhances IgE - mediated allergy in elderly patients with asthma.