Abstract : Influence of cigarette smoking on leukotriene 84 (LTB4) generation by leucocytes associated with bronchial hyperresponsiveness was examined in 110 patients with asthma. l. The frequency of positive RAST score for inhalant allergens and LTB4 generation by leucocytes were significantly larger in previous and current smokers than in nonsmokers. 2. In nonsmokers, bronchial hyperresponsiveness was significantly higher in asthmatics under the age of 49 years compared with that in patients between 50 and 59 years, between 60 and 69 years, and those over age 70 years. In contrast, no significant differences were present between bronchial hyperresponsiveness and patient age in previous and current smokers. 3. In patients over age 60, bronchial hyperresponsiveness was significantly higher in previous and current smokers than in nonsmokers. In these patients, LTB4 generation was significantly more increased in previous and current smokers than in nonsmokers. 4. A significant correlation was observed in asthmatics of nonsmokers between LTB4 generation and bronchial hyperresponsiveness . 5. LTC4 generation was significantly higher in previous and current smokers than in nonsmokers in patients under age 49 years. These results show that in patients over age 60 years, cigarette smoking induces an increase in the generation of LTB4, leading to an increase in bronchial hyperresponsiveness .