Recently, the number of patients with Japanese cedar (Cryptomeria japonica) pollinosis has increased, especially in children. However, little is known about the incidence in infants. We studied on the rate of sensitization and the onset of pollinosis in children under 6 years old. The percentage of positive CAP-RAST to Japanese cedar pollen was 27.6%, in 76 infants (51 male and 25 female, 2 months-5 years old) who visited National Mie Hospital pediatric allergy clinic due to bronchial asthma and/or atopic dermatitis. The youngest child who has been sensitized to pollen was 1 year 8 month old boy. The percentage of positive rate of CAP-RAST to house dust mite was 61.8%. Twenty-seven infants (20 male and 7 female, 2-5 years of age) were diagnosed as Japanese cedar pollinosis in National Mie Hospital Otorhinolaryngology clinic in 1999 and 2000. The youngest child with pollinosis was 2 year 5 month old boy. Most of the 27 infants complained of rhinorrhea and/or eye symptoms and some of them complained cough, snoring, or epistaxis. About 40% were sensitized to Japanese cedar and/or cupressaceae pollen alone, 60% were also sensitized to house dust mite. In conclusion, it is possible that the sensitization to Japanese cedar pollen occurs after 2 season of pollen exposure and pollinosis occurs in 2 years old. Japanese cedar pollen has been an important allergen not only in school children, but also in infants.