The cumulated incidence of atopic disease before 7 years of age was highly influenced by the cord blood IgE concentration and moderately influenced by the month of birth: obvious atopic disease was more than twice as common in children with high cord blood IgE and born in May than in children with a similar IgE and born in November (P<0.05). There was no similarly increased risk for atopic disease among those born in May with low cord blood IgE. Environmental factors thus seem important merely for a high risk population. The findings lend support to family planning when there is especially high genetic risk for atopic disease.