Pranlukast (PLK) is a cysteinyl leukotriene receptor 1 antagonist approved for the treatment of bronchial asthma and allergic rhinitis in Japan. We previously reported that PLK dry syrup (DS) improved the total nasal symptom score, as well as sneezing, nasal discharge, and nasal obstruction scores over placebo. We investigated the efficacy of PLK DS with a noninvasive method in 10-to 15-year-old children with Japanese cedar (JC) pollinosis challenged with pollen allergen using an artificial exposure chamber (OHIO Chamber). Levels of eosinophil cationic protein (ECP) in nasal secretions, nasal obstruction score, and the relationship with nasal obstruction scores were analyzed. The estimated difference of means in ECP levels (PLK DS-placebo) was-22.9 micrograms (95% CI,-45.2 to-0.5), suggesting PLK DS reduced ECP significantly when compared with placebo (p= 0.0454). The difference in the least square means for nasal obstruction between the PLK DS and placebo was-0.25 (95% CI,-0.36 to-0.14) with a value of p < 0.0001. In addition, a statistically significant, although weak, positive correlation between the nasal obstruction score and nasal ECP levels was observed with placebo treatment (correlation coefficient= 0.2394; p= 0.0428). Moreover, the inhibition rate of nasal ECP with PLK DS relative to placebo was statistically significant, although weak, positively correlated with the inhibition rate of nasal obstruction (correlation coefficient= 0.3373; p= 0.0219). PLK DS significantly decreases nasal ECP levels and nasal obstruction score compared with placebo in children with JC pollinosis challenged with pollen allergen. Suppression of mucosal eosinophilic inflammation is one of the pathways by which PLK DS improves pollinosis-induced nasal obstruction.