Since the cyclooxygenase (COX) and 5-lipoxygenase (5-LO) pathway share arachidonic acid cascade (CAAC) as a common substrate, blockade of the prostanoid pathway by NSAIDs was proposed to shunt arachidonate down the 5-LO pathway, leading to elevated formation of cys-LTs and hence to an adverse respiratory reaction.<BR>We investigated whether inhaled Saiboku-to (Japanese herbal medicine, Cai-pu-tang : CPT) attenuates the response to bronchial challenge with L-lysine aspirin (LLA) and the associated increase in LTC₄ and LTD₄ in bronchoalveolar lavage fluid (BALF) in 74 bronchial asthmatic patients with aspirin-induced asthma (AIA). Each subject was challanged twice with a single threshold dose of LLA, which caused a FEV_1.0 of 20% or more decrease in a preliminary test, immediately after inhation of various concentrations of mainly 1, 000 μg/ml CPT in 5 ml physiological saline or the same dose of physiological saline as placebo in a randomized double-blind controlled protocol. FEV1.0 was recorded at 30 min intervals for 4h. LTC₄ and LTD₄ in BALF and LTE4 in urine were measured by combined high-performance liquid chromatography and radioimmunoassay. After placebo, LASA caused an obstructive reaction in all patients, both LTC₄ and LTD₄ in BALF and LTE₄ in urine increased after challenge. Preinhaled CPT provided almost completed protection against bronchoconstriction by challange in all patients. Increased LTC₄ and LTD₄ in BALF by challenge with LLA was also suppressed. This data confirms that preinhalation of CPT is highly effective in preventing AIA and sugggests that this effect is mediated by inhibition of LT production. Inhalation of placebo or CPT for 6 months as 100 μg/ ml×5ml/3 times/day in a randomized double-blind test was done in 74 patients with AIA. Inhalation of CPT significantly improved quality of life (QOL), e. g., total-diseases-related symptoms (DRS), its components, the QWB, sore, face scale (vs before p < 0.01, vs placebo p<0.01), and symptoms induced by AIA (vs before : p<0.01, vs placebo p<0.01). These improvements of AIA and QOL are discussed.