Effects of spa therapy on asthma were studied in 64 patients with asthma in relation to the generation of leukotrienes B4 (LTB4) and C4 (LTC4) and bronchial hyperresponsiveness. 1. The efficacy of spa therapy was marked in 12 (18.8%), and moderate in 45 (70.3%) of 64 patients with asthma, and slight or no efficacy of the therapy was observed in the residual 7 patients (10.9%). 2. Bronchial hyperresponsiveness to methacholine was the highest in patients with slight or no efficacy of spa therapy, however, there were no significant differences among the three groups classified by clinical efficacy. 3. The generation of LTC4 by leucocytes was significantly higher in patients with slight or no efficacy compared with the generation in those with marked (p<0.01) and moderate efficacy (p<0.001). However, there were no significant differences in the generation of LTB4 among them. 4. The generation of LTC4 before spa therapy significantly decreased in patients with marked (p<0.001) and moderate efficacy (p<0.01) after spa therapy, but not in those with slight or no efficacy. The generation of LTB4 was not significantly different before and after spa therapy among patients with marked, moderate, and slight or no efficacy. These results show that the efficacy of spa therapy for patients with asthma is closely related to the generation of LTC4 by leucocytes, and that the generation of LTC4 significantly decreases after spa therapy in patients with spa efficacy.