The generation of leukotrienes B4 (LTB4) and C4 (LTC4) by leucocytes stimulated with Ca ionophore A23187 was examined in 67 patients with asthma. 1. The generation of leukotriene B4 (LTB4) by leucocytes was significantly more increased in patients with asthma than in those with pulmonary emphysema (PE) and healthy subjects. The generation of leukotriene C4 (LTC4) was also signficantly more increased in patients with asthma compared to the generation in those with PE and healthy subjects. The generation of both LTB4 and LTC4 was significantly more increased in patients with PE than in healthy subjects. 2. The generation of both LTB4 and LTC4 was larger in patients with asthma over age 70 than in those of other age groups, however, the difference was not significant. 3. The generation of LTC4 was significantly higher in attack stage than in non-attack stage in all age groups of patients with asthma, however, the generation of LTB4 was not significantly different between the two stages. 4. Bronchial hyperresponsiveness to methacholine in patients with asthma tended to decrease with aging, and the bronchial hyperresponsiveness was to a certain extent correlated with the generation of LTB4, but not LTC4. 5. The generation of LTC4 was correlated with value of FEV1.0%. These results may suggest that both LTB4 and LTC4 participate in the pathophysiology of asthma in the elderly.