Recently, increases in cases of Japanese cedar pollinosis have been found, especially in children. School children, junior high school students and high-school students in Mibu-machi, Tochigi prefecture were given questionnaires, rhinoscopic examinations were done and nasal secretions and blood samples were taken. The following results for the process of sensitization and the onset of Japanese cedar pollinosis were found. According to the questionnaire, the percentage of positive Japanese cedar pollinosis was about 10% in 6-year-olds and about 25% in 17-year-olds. However, according to the blood samples, Japanese cedar pollen antibodies abruptly increased in 7-year-olds. We found age risk factors. Sensitization to Japanese cedar pollinosis appeared in 7-year-old, with symptoms of pollinosis developing in 8-year-olds. We compared the percentage of positive rate of antibodies of Japanese cedar pollen with percentage of Dermatophagoides farinae (Der f) from the student's blood samples. Children having Der f antibodies showing higher positive rate of Japanese cedar pollen antibodies than children who do not have Der f antibodies. Sensitization to Japanese cedar pollen is dependent on Der f. We paid attention to resident environments. Children having Japanese cedar pollen antibodies were higher in children living in the urban than in rural. The percentage of onset of symptoms was higher in apartment buildings than in detached houses. In conclusion, sensitization to Japanese cedar pollen occurs mostly at age 6 - 7, is influenced by type of dwelling (apartment or house), (specially by Der f) and it's onset is affected by urban or rural.