The correlation between the efficacy of 4-weeks administration with pranlukast, leukotriene receptor antagonist, and LTs generation by peripheral leukocytes were evaluated in 18 patients with mild-persistent asthma. The efficacy of pranlukast administration was assessed by symptom, morning PEF and pulmonary function. Pranlukast were effective in 12/18(67%) patients. In those patients, LTC4 generation before pranlukast administration was significantly high, compared with that in pranlukast-ineffective patients. LTC4 generation decreased after 4-weeks administration with pranlukast in effective patients. In ineffective patients, however, LTC4 generation increased after 4-weeks administration. LTB4 had shown no significant difference between effective and ineffective patients before administration, and LTB4 decreased after 4-weeks in both groups. Proport ion of peripheral eosinophi Is in effective patients were higher than that in ineffective patients, however not significant. After 4-weeks, proportion of eosinophi Is was decreased in effective patients and increased in ineffective patients. These findings suggest that pranlukast is effective for patients with high LTC4 generation and has the effect to suppress the accumulation of eosinophils in such patients.