The effect of exposure to Japanese cedar pollen (JCP) in early life on subsequent sensitization to it was evaluated. Specific IgE antibody to JCP was examined in 440-504 school children in a rural town each year during 1995-98. The amount of dispersed pollen measured by a Durham sampler widely ranged from 165 to 5941 grains/cm2/year during this period. The amount had been measured during the period of 1982-91 in which these children were born, and it also widely ranged from 148 to 8566 grains/cm2/year. Children born during November to January, who were exposed to JCP within 6 months of age, increased at the risk of sensitization to JCP, especially severe sensitization, relative to those born in the other months. Age-adjusted prevalence rate ratio (RR) of having a JCP-IgE*15 U/ml (control; <0.35 U/ml) for children born in December to February relative to children born in the other months was 1.74 (95% confidence interval; 1.06-2.87, examined in 1998), and for those born in November to January was 1.57 (95% CI; 1.00-2.46, examined in 1997). The risk of sensitization to JCP was low for those born in May to July (RR=0.42, 95%Cl; 0.19-0.93, examined in 1998). There was also a strong correlation between the amount of the dispersed pollen during the period of 2-6 months after birth and the prevalence of sensitization to JCP. J Epidemiol, 2000; 10: 42-47