In 1993 extremely high levels of birch pollen were recorded in Stockholm, Sweden. We investigated the effects of this exposure on sensitization and development of atopic airway disease in children. The aim of this study was to assess the influence of maternal birch sensitization and symptoms of pollen allergy, as well as exposure to birch pollen during pregnancy, on sensitization and development of atopic airway disease in children. A total of 387 children with atopic heredity (70% had atopic mothers) and born in Stockholm 1993 or 1994 were investigated at age 4.5-5 yr. The children were examined and skin prick tested with inhalant and food allergens. IgE-antibodies against birch pollen and recombinant birch pollen allergen were analyzed in serum. The same tests were performed on the mothers. Children of mothers with symptoms of pollen allergy more often showed symptoms of rhinoconjunctivitis at age 4.5-5, after both high dose [Odds ratio (OR) 5.3; 95% confidence interval (CI): 2.0-13.7] and low dose (OR 4.0; 95% CI: 1.5-10.9) exposure to birch pollen during pregnancy. Similar tendencies were noted for children of mothers sensitized to birch, where stronger effects were suggested in boys (OR 3.8; 95% CI: 1.3-11.5) than in girls (OR 1.2; 95% CI: 0.2-5.5) in the high-dose exposed group. For asthma symptoms and sensitization to birch in the children the results were less consistent. It may be concluded that, maternal pollen allergy seems to have a stronger influence on the development of rhinoconjunctivitis in children with a family history of atopy than the degree of allergen exposure during pregnancy.