Background It has been suggested that allergic sensitization is inversely related to the number of siblings in the family.
Objectives To study whether a similar relation can be observed for hay fever and asthma among Finnish adolescents in a population with relatively low prevalence of atopic diseases.
Methods A questionnaire mailed to a nationwide sample of 1849 families with 16-year- old twins assessing the cumulative incidence of doctor-diagnosed hay fever and asthma among the adolescents and the number of older siblings in the family by parental report.
Results The cumulative incidence of hay fever was significantly lower among the adolescents with three or more older siblings (3.9%, 95% CI= 1.2–6.5%) compared with adolescents with fewer older siblings (12.7%, 95% CI=11.4–14.0%). There was no difference in the cumulative incidence of asthma among the adolescents according to the number of older siblings in the family.
Conclusions Large number of older siblings appears to be protective against the development of hay fever.