BACKGROUND: Few studies have focused on pollen exposure and asthma in children. None have examined associations between persistent exposure to pollen in infancy and aeroallergen sensitisation and asthma in childhood. OBJECTIVES: To examine the association between higher ambient levels of pollen in the first 3-6 months of life and risk of eczema, sensitization to food and aeroallergens at 2 years and asthma or hayfever at age 6-7 years combined. METHODS: Using a birth cohort of 620 infants with a family history of allergic disease born between 1990 and 1994, we examined risk of eczema or allergic sensitization (SPT > 3 mm to at least one of cow's milk, egg white, peanut, house dust-mite, rye grass, and cat dander) by age 2 and asthma or hayfever at age 6-7. Daily ambient levels of pollen were measured during this period. RESULTS: Cumulative exposure to pollen concentrations up to 6 months was associated with aeroallergen sensitization with the highest risk occurring at 3 months (aOR = 1.34, 95% CI 1.06-1.72). Cumulative exposure to pollen up to 3 months was also associated with hayfever (aOR = 1.14, 95% CI 1.009-1.29) and between 4 and 6 months exposure with asthma only (aOR=1.35, 95% CI 1.07-1.72). CONCLUSION: Persistent pollen exposure in infancy appears to increase the risk of asthma and hayfever in children. These results support the hypothesis that there is a critical window of opportunity in early development which may be important for modification of allergic outcomes.