Changes in the generation of leukotrienes B4 (LTB4) and C4 (LTC4) by leucocytes after spa therapy were examined in 30 patients with asthma. 1. The efficacy of spa therapy was marked in 8 (26.7%), and moderate in 16 (53.3%) of 30 patients with asthma, and slight or no efficacy of the therapy was observed in the residual 6 patients (20.0%). 2. Bronchial reactivity 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 was significantly higher in patients with slight or no efficacy compared with the generation in those with marked (p