Dear Editor, Japanese cedar (Cryptomeria japonica) pollen (JCP) and Japanese cypress (Chamaecyparis obtusa) pollen (JCyP) are most prevalent pollen allergen in Japan. Dispersal of JCP generally precedes that of JCyP, but the pattern of each is typically arch-shaped over time. JCP dispersal commences in early February and peaks between late February and early March, whereas dispersal of JCyP peaks between late March and early April, with some variation due to yearly climatic changes. In and around Tokyo, the pollen dispersal season is in excess of 10 weeks. Many patients with JCP-induced AR also have known sensitivity to JCyP, given their shared allergenicity. AR triggered by JCyP alone is thus a rarity. Because dispersal periods of the two pollen species do overlap, AR due exclusively to JCyP has been difficult to characterize. Through its antecedent or concurrent dispersal, JCP may well impact the symptoms and severity of JCyP-induced AR. Environmental challenge chamber (ECC) enables selective introduction of various pollen types at constant levels and under identical conditions, allowing highly reproducible clinical evaluations.