Japanese cedar pollinosis has shown large increases especially in city areas, rather than in the rural, with increases being remarkable especially among people of young age. In order to elucidate factors for sensitization and onset of Japanese cedar pollinosis, a questionnaire survey was conducted and serum IgE antibody levels examined in junior high school students living in three different areas, i.e., city, seaside and mountainous areas in Aichi prefecture. Prevalence rate of Japanese cedar pollinosis, estimated from the number of students having both nasal or ocular allergic symptoms and positive RAST, was the 30.1% in the mountainous area with high Japanese cedar pollen count, being greater than the 15.3% in the city area and the 5.8% in the seaside area, both with low pollen counts. Among the students showing high serum mite (Dermatophagoides farinae) antibody levels, greater positive rates for sensitization to Japanese cedar or orchard grass pollens was observed, compared to mite-unsensitized students. This result suggests that sensitization to seasonal antigens may be corresponding to the capability for sensitization to the mites. Besides this finding, the analysis of replies to the questionnaires showed that students living in concrete or steel-frame buildings had greater serum levels of antibody to Japanese cedar pollen than those living in wood buildings, indicating a possible relationship between the pollen sensitization and indoor environments.