Abstract Objective: The purpose of the present study was to investigate the effectiveness of chemosurgery with 80 w/v% trichloroacetic acid (TCA) for the treatment of Japanese cedar pollenosis. The effectiveness of this treatment was evaluated in terms of the levels in the nasal washings of the chemical mediators histamine and eosinophil cationic protein, and the reactivity in the nasal provocation test. Methods: Patients of cedar pollenosis were divided into two groups, the TCA-treated group ( n = 90) and the group that was not treated with TCA (nonTCA-treated group) ( n = 79), according to whether or not they received TCA treatment. In both the groups, the levels of the two aforementioned mediators in the nasal washings were measured during the pollen dispersal season. In addition, allergen provocation tests were performed using the disk method in volunteers from both the groups during the non-pollen dispersal season. The results of the above two determinations were compared statistically between the two groups. Results: The results revealed significantly lower levels of the two mediators in the TCA-treated group than in the nonTCA-treated group ( p < 0.01). There was also a greater tendency for the subjects in the TCA-treated group as compared to those in the nonTCA-treated group to show negative reactivity in the allergen provocation test ( p < 0.01). Conclusion: Regional suppression of the allergic reaction to Japanese cedar pollen appears to occur as a result of chemosurgery with TCA performed as a day surgery.