We report on the relation between the month of birth and the chance of developing an IgE antibody response as found in a study sample of 150,000 subjects. Our results confirm that for the three seasonal allergens birch pollen, grass pollen and house dust mite, an increased relative risk was found for subjects born up to 3 months before the main season for that allergen in The Netherlands. For cat and dog allergy an increased relative risk was found from November to January, perhaps reflecting increased exposure to these pets during the winter. Surprisingly, however, also for egg white and cow's milk a clearly increased relative risk was found from November to January and a decreased relative risk in May. These data support the hypothesis of a 'sensitive' period in the first months of life during which allergen exposure is more likely to prime for an allergy later in life. The results with the non-seasonal allergens suggest that another seasonal factor exists which early in life assists (or prevents) priming by allergen.